boards/esp32p4: Remove unnecessary CONFIG_ESPRESSIF_MERGE_BINS

[JianyuWang0623]

Note: Please adhere to Contributing Guidelines.

Summary

ESP32-P4 boards use simple boot mode and do not have a separate
second-stage bootloader, so there is no need to merge multiple
binaries into a single padded image. The CONFIG_ESPRESSIF_MERGE_BINS
option causes the build to produce a 16 MB padded image solely for
unified flashing, which is unnecessary on these configs.

This patch removes CONFIG_ESPRESSIF_MERGE_BINS=y from the following
three esp32p4-function-ev-board defconfigs:

• configs/ethernet/defconfig
• configs/i2schar/defconfig
• configs/spislv/defconfig

After the change, make flash writes only the actual firmware
(~380 KB) instead of the 16 MB padded image.

Impact

• User: make flash is significantly faster on the affected
  configs (~55 s -> ~4.5 s).
• Build: No more nuttx.merged.bin is produced for these
  configs; the flashed artifact is nuttx.bin directly.
• Hardware: ESP32-P4-Function-EV-Board only. Other ESP chips
  (ESP32, ESP32-S3, ESP32-C3, etc.) that rely on a true second-stage
  bootloader are not touched.
• Documentation / Security / Compatibility: None.

Testing

Host: Ubuntu Linux, x86_64
Board: ESP32-P4-Function-EV-Board

1. Refreshed all three modified defconfigs with the upstream tool
   to make sure they are still normalized:

   $ ./tools/refresh.sh --silent esp32p4-function-ev-board:ethernet
     [1/1] Normalize esp32p4-function-ev-board:ethernet
   $ ./tools/refresh.sh --silent esp32p4-function-ev-board:i2schar
     [1/1] Normalize esp32p4-function-ev-board:i2schar
   $ ./tools/refresh.sh --silent esp32p4-function-ev-board:spislv
     [1/1] Normalize esp32p4-function-ev-board:spislv
   $ git status --short
   (no diff produced for any of the three defconfigs)
   

2. Built and flashed each config; observed make flash writing only
   nuttx.bin (~380 KB) and the flashing time dropping from ~55 s
   to ~4.5 s. The boards boot normally and the corresponding
   functional tests (NSH, i2schar, spislv, ethernet ping) still
   pass.