b-g474e-dpow1: Add support for FLASH bootloader

boards/arm/stm32/b-g474e-dpow1/scripts/ld.script.dfu:

    * New file. Reserve up to 24KiB at the start of FLASH for a
      bootloader.

boards/arm/stm32/b-g474e-dpow1/scripts/Make.defs:

    * When building with CONFIG_STM32_DFU, use the new ld.script.dfu
      to leave room in FLASH for a bootloader.

boards/arm/stm32/b-g474e-dpow1/README.txt:

    * "FLASH Bootloader Support": New section.

boards/arm/stm32/b-g474e-dpow1/README.txt

@@ -22,6 +22,7 @@ Contents
     - LEDs
     - RGB Power LED
   - Serial Consoles
+  - FLASH Bootloader Support
   - Configurations
 
 
@@ -114,6 +115,28 @@ Serial Consoles
   differ.
 
 
+FLASH Bootloader Support
+========================
+
+  If implementing a FLASH bootloader, turn on Kconfig option CONFIG_STM32_DFU.
+  This option activates an alternate linker script, scripts/ld.script.dfu,
+  which causes NuttX to leave a gap at the start of FLASH, leaving that space
+  for the FLASH bootloader. See scripts/ld.script.dfu for details. It also
+  causes NuttX to relocate its vector table and possibly make other
+  adjustments.
+
+  One possible bootloader is STmicro's OpenBootloader "middleware" supplied
+  with STM32CubeG4 version 1.3.0. On the host (PC), it should be possible to
+  use STmicro's STM32CubeProgrammer or the stm32loader.py script from
+  https://github.com/jsnyder/stm32loader. That script can be invoked with
+  parameters such as:
+
+    stm32loader.py -p /dev/ttyACM0 -a 0x08006000 -e -w -v -g 0x08006000 nuttx.bin
+
+  where the given address (0x08006000 in this case) must match the starting
+  address in scripts/ld.script.dfu.
+
+
 Configurations
 ==============
 

boards/arm/stm32/b-g474e-dpow1/scripts/Make.defs

@@ -22,12 +22,18 @@ include $(TOPDIR)/.config
 include $(TOPDIR)/tools/Config.mk
 include $(TOPDIR)/arch/arm/src/armv7-m/Toolchain.defs
 
+ifeq ($(CONFIG_STM32_DFU),y)
+  LDSCRIPT = ld.script.dfu
+else
+  LDSCRIPT = ld.script
+endif
+
 ifeq ($(CONFIG_CYGWIN_WINTOOL),y)
   # Windows-native toolchains
-  ARCHSCRIPT = -T "${shell cygpath -w $(BOARD_DIR)$(DELIM)scripts$(DELIM)ld.script}"
+  ARCHSCRIPT = -T "${shell cygpath -w $(BOARD_DIR)$(DELIM)scripts$(DELIM)$(LDSCRIPT)}"
 else
   # Linux/Cygwin-native toolchain
-  ARCHSCRIPT = -T$(BOARD_DIR)$(DELIM)scripts$(DELIM)ld.script
+  ARCHSCRIPT = -T$(BOARD_DIR)$(DELIM)scripts$(DELIM)$(LDSCRIPT)
 endif
 
 ifeq ($(CONFIG_DEBUG_SYMBOLS),y)

boards/arm/stm32/b-g474e-dpow1/scripts/ld.script.dfu

@@ -0,0 +1,127 @@
+/****************************************************************************
+ *  boards/arm/stm32/b-g474e-dpow1/scripts/ld.script
+ *
+ *  Licensed to the Apache Software Foundation (ASF) under one or more
+ *  contributor license agreements.  See the NOTICE file distributed with
+ *  this work for additional information regarding copyright ownership.  The
+ *  ASF licenses this file to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance with the
+ *  License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ *  License for the specific language governing permissions and limitations
+ *  under the License.
+ *
+ ****************************************************************************/
+
+/* The STM32G474RE has 512 KiB of FLASH beginning at address 0x0800:0000.
+ *
+ * When booting from FLASH, FLASH memory is aliased to address 0x0000:0000
+ * where the code expects to begin execution by jumping to the entry point in
+ * the 0x0800:0000 address range. The FLASH bootloader is located there and
+ * allocated up to 24KiB (6 pages of 4k if single bank mode or 12 pages of 2k
+ * if dual bank mode), so our executable will begin at 0x0800:6000, leaving
+ * 488KiB.
+ *
+ * The STM32G474RE has a total of 128 KiB of SRAM in three separate areas:
+ *
+ * 1) 80 KiB SRAM1 mapped at 0x2000:0000 thru 0x2001:3fff.
+ * 2) 16 KiB SRAM2 mapped at 0x2001:4000 thru 0x2001:7fff.
+ *
+ * CCM SRAM (Routine Booster):
+ *
+ * 3) 32 KiB CCM SRAM mapped at 0x1000:0000 thru 0x1000:7fff
+ *    but also aliased at at 0x2001:8000 thru 0x2001:ffff to be contiguous
+ *    with the SRAM1 and SRAM2.
+ *
+ * Because SRAM1 and SRAM2 are contiguous, they are treated as one region
+ * by this logic.
+ *
+ * CCM SRAM is also contiguous to SRAM1 and SRAM2, however it is excluded
+ * from this linker script, to keep it reserved for special uses in code.
+ * REVISIT: Is this the correct way to handle CCM SRAM?
+ */
+
+MEMORY
+{
+    flash (rx) : ORIGIN = 0x08006000, LENGTH = 488K
+    sram (rwx) : ORIGIN = 0x20000000, LENGTH = 96K
+}
+
+OUTPUT_ARCH(arm)
+EXTERN(_vectors)
+ENTRY(_stext)
+
+SECTIONS
+{
+    .text : {
+        _stext = ABSOLUTE(.);
+        *(.vectors)
+        *(.text .text.*)
+        *(.fixup)
+        *(.gnu.warning)
+        *(.rodata .rodata.*)
+        *(.gnu.linkonce.t.*)
+        *(.glue_7)
+        *(.glue_7t)
+        *(.got)
+        *(.gcc_except_table)
+        *(.gnu.linkonce.r.*)
+        _etext = ABSOLUTE(.);
+    } > flash
+
+    .init_section : ALIGN(4) {
+        _sinit = ABSOLUTE(.);
+        *(.init_array .init_array.*)
+        _einit = ABSOLUTE(.);
+    } > flash
+
+    .ARM.extab : ALIGN(4) {
+        *(.ARM.extab*)
+    } > flash
+
+    .ARM.exidx : ALIGN(4) {
+    __exidx_start = ABSOLUTE(.);
+        *(.ARM.exidx*)
+        __exidx_end = ABSOLUTE(.);
+    } > flash
+
+    _eronly = ABSOLUTE(.);
+
+    .data : ALIGN(4) {
+        _sdata = ABSOLUTE(.);
+        *(.data .data.*)
+        *(.gnu.linkonce.d.*)
+        CONSTRUCTORS
+        . = ALIGN(4);
+        _edata = ABSOLUTE(.);
+    } > sram AT > flash
+
+    .bss : ALIGN(4) {
+        _sbss = ABSOLUTE(.);
+        *(.bss .bss.*)
+        *(.gnu.linkonce.b.*)
+        *(COMMON)
+        . = ALIGN(4);
+        _ebss = ABSOLUTE(.);
+    } > sram
+
+    /* Stabs debugging sections. */
+
+    .stab 0 : { *(.stab) }
+    .stabstr 0 : { *(.stabstr) }
+    .stab.excl 0 : { *(.stab.excl) }
+    .stab.exclstr 0 : { *(.stab.exclstr) }
+    .stab.index 0 : { *(.stab.index) }
+    .stab.indexstr 0 : { *(.stab.indexstr) }
+    .comment 0 : { *(.comment) }
+    .debug_abbrev 0 : { *(.debug_abbrev) }
+    .debug_info 0 : { *(.debug_info) }
+    .debug_line 0 : { *(.debug_line) }
+    .debug_pubnames 0 : { *(.debug_pubnames) }
+    .debug_aranges 0 : { *(.debug_aranges) }
+}