boards/*/esp32: Add instructions on how to use the configs with QEMU.

Signed-off-by: Abdelatif Guettouche <abdelatif.guettouche@espressif.com>

boards/risc-v/esp32c3/esp32c3-devkit/README.txt

@@ -49,7 +49,7 @@ partition table, these are then presented as Github assets and can be downloaded
 from: https://github.com/espressif/esp-nuttx-bootloader/releases
 Download bootloader-esp32c3.bin and partition-table-esp32c3.bin and place them
 in a folder, the path to this folder will be used later to program them. This
-can be: "../esp-bins/esp32c3/"
+can be: "../esp-bins"
 
 Buttons and LEDs
 ================
@@ -111,8 +111,8 @@ included in the build process.
 The esptool.py command to flash all the binaries is `esptool.py --chip esp32c3
 --port /dev/ttyUSBXX --baud 921600 write_flash 0x0 bootloader.bin 0x8000 partition-table.bin 0x10000 nuttx.bin`
 However, this is also included in the build process and we can use build and flash with:
-`make download ESPTOOL_PORT=/dev/ttyUSBXX ESPTOOL_BINDIR=../esp-bins/esp32c3`
-The "../esp-bins/esp32c3" path is the path to the folder containing the bootloader and the
+`make download ESPTOOL_PORT=/dev/ttyUSBXX ESPTOOL_BINDIR=../esp-bins`
+The "../esp-bins" path is the path to the folder containing the bootloader and the
 partition table for the ESP32-C3 as explained above.
 Note that this step is required only one time.  Once the bootloader and partition
 table are flashed, we don't need to flash them again.  So subsequent builds

boards/xtensa/esp32/esp32-devkitc/README.txt

@@ -858,6 +858,20 @@ NOTES:
 
     Where x in the watchdog instance.
 
+Using QEMU:
+==========
+
+First follow the instructions at https://github.com/espressif/qemu/wiki to build QEMU.
+Enable the ESP32_QEMU_IMAGE config found in "Board Selection -> ESP32 binary image for QEMU".
+Download the bootloader and the partition table from https://github.com/espressif/esp-nuttx-bootloader/releases
+and place them in a directory, say ../esp-bins.
+Build and generate the QEMU image: `make ESPTOOL_BINDIR=../esp-bins`
+A new image "esp32_qemu_image.bin" will be created.  It can be run as:
+
+ ~/PATH_TO_QEMU/qemu/build/xtensa-softmmu/qemu-system-xtensa -nographic \
+    -machine esp32 \
+    -drive file=esp32_qemu_image.bin,if=mtd,format=raw
+ 
 Things to Do
 ============
 

boards/xtensa/esp32/esp32-ethernet-kit/README.txt

@@ -67,3 +67,17 @@ Espressif has an offcial Ethernet development board:
 
   https://docs.espressif.com/projects/esp-idf/en/latest/esp32/hw-reference/esp32/get-started-ethernet-kit.html
 
+Using QEMU:
+==========
+
+First follow the instructions at https://github.com/espressif/qemu/wiki to build QEMU.
+Enable the ESP32_QEMU_IMAGE config found in "Board Selection -> ESP32 binary image for QEMU".
+Download the bootloader and the partition table from https://github.com/espressif/esp-nuttx-bootloader/releases
+and place them in a directory, say ../esp-bins.
+Build and generate the QEMU image: `make ESPTOOL_BINDIR=../esp-bins`
+A new image "esp32_qemu_image.bin" will be created.  It can be run as:
+
+ ~/PATH_TO_QEMU/qemu/build/xtensa-softmmu/qemu-system-xtensa -nographic \
+    -machine esp32 \
+    -drive file=esp32_qemu_image.bin,if=mtd,format=raw
+ 

boards/xtensa/esp32/esp32-wrover-kit/README.txt

@@ -77,3 +77,18 @@ Configurations
     nsh> gpio -w 14 /dev/gpint3
   The pin is configured to as a rising edge interrupt, so after issuing the
   above command, connect it to 3.3V.
+
+Using QEMU:
+==========
+
+First follow the instructions at https://github.com/espressif/qemu/wiki to build QEMU.
+Enable the ESP32_QEMU_IMAGE config found in "Board Selection -> ESP32 binary image for QEMU".
+Download the bootloader and the partition table from https://github.com/espressif/esp-nuttx-bootloader/releases
+and place them in a directory, say ../esp-bins.
+Build and generate the QEMU image: `make ESPTOOL_BINDIR=../esp-bins`
+A new image "esp32_qemu_image.bin" will be created.  It can be run as:
+
+ ~/PATH_TO_QEMU/qemu/build/xtensa-softmmu/qemu-system-xtensa -nographic \
+    -machine esp32 \
+    -drive file=esp32_qemu_image.bin,if=mtd,format=raw
+