arch/risc-v: update MCUboot slot sizes on Espressif devices

[fdcavalcanti]

Summary

RISC-V Espressif Kconfig defaults for MCUboot OTA layout are updated to use slot size 0x150000 (~1.4MB), with secondary slot offset 0x170000, scratch at 0x2C0000, storage MTD at 0x300000, and matching documentation updates on the ESP32-C3, ESP32-C6, and ESP32-H2 platform pages.

• arch/risc-v: update MCUBoot slot size and offsets

Increase image slot size from ~1MB to ~1.4MB to allow more features by default.
Solves issues when enabling debug features would extrapolate the slot size.

• documentation: update ESP32-C3|C6|H2 flash allocation table

Update the user documentation for flash allocation when using MCUboot.

Reason for this change: image size is getting to close to 1MB and may not fit the slot when some features such as Wi-Fi are enabled.

Impact

Impact on user: No.

Impact on build: Yes, avoids bad image build when too many features are enabled along MCUboot.

Impact on hardware: ESP32-C3|C6|H2|P4.

Impact on documentation: Yes, updates the documentation for flash allocation when using MCUboot.

Impact on security: No.

Impact on compatibility: No.

Testing

Building

• ./tools/configure.sh esp32c6-devkitc:mcuboot_update_agent
• Enable DEBUG_ASSERTIONS and DEBUG_FEATURES
• make bootloader
• make ESPTOOL_BINDIR=./

Results

• Before change:
  Build would failure when imgtool was invoked because the image size was bigger than the MCUboot slot.

Usage: imgtool sign [OPTIONS] INFILE OUTFILE
Try 'imgtool sign -h' for help.

Error: Image size (0x10a894) + trailer (0x630) exceeds requested size 0x100000
make: *** [tools/Unix.mk:575: nuttx] Error 2

• After changes:
  Build succeeds.