docs and comments: Update stale references to COPYING

Since 7a046358d9 the top-level COPYING
file has been deleted and replaced by DISCLAIMER, LICENSE, and NOTICE
files. However, some references to the old COPYING file remained in
Kconfig help text and documentation.

Documentation/contributing/coding_style.rst:
Documentation/introduction/about.rst:
boards/arm/lpc17xx_40xx/olimex-lpc1766stk/README.txt:
boards/arm/sam34/arduino-due/README.txt:
boards/arm/sam34/sam4l-xplained/README.txt:
boards/arm/sama5/giant-board/README.md:
boards/arm/sama5/sama5d2-xult/README.txt:
boards/arm/sama5/sama5d4-ek/README.txt:
boards/arm/samd2l2/samd20-xplained/README.txt:
boards/arm/samd2l2/samd21-xplained/README.txt:
boards/arm/samd2l2/saml21-xplained/README.txt:
boards/arm/stm32/hymini-stm32v/README.txt:
boards/arm/stm32/stm3210e-eval/README.txt:
fs/fat/Kconfig:
libs/libc/string/Kconfig:

    * Updates stale references to the old top-level COPYING file to
      either LICENSE or NOTICE (or both), as appropriate in each
      instance.

boards/arm/sam34/arduino-due/README.txt

@@ -788,7 +788,7 @@ Configuration sub-directories
          CONFIG_FAT_MAXFNAME=32            : Maximum supported file name length
 
          There are issues related to patents that Microsoft holds on FAT long
-         file name technologies.  See the top level COPYING file for further
+         file name technologies.  See the top level NOTICE file for further
          details.
 
        Device Drivers

boards/arm/sam34/sam4l-xplained/README.txt

@@ -503,7 +503,7 @@ Configuration sub-directories
          CONFIG_FAT_MAXFNAME=32            : Maximum supported file name length
 
          There are issues related to patents that Microsoft holds on FAT long
-         file name technologies.  See the top level COPYING file for further
+         file name technologies.  See the top level NOTICE file for further
          details.
 
        System Type -> Peripherals: