Update bootloader so flash mode can be notify

[karliatto]

When the ESP32 is in flash mode (keep pressed flash button and then press boot) the bootloader code is running.
It would be useful to let the user know the flash mode is on, in the most simple way to add a let to the PCB that blinks when flash mode is on.

[chill117]

Reference links:

• https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-guides/bootloader.html
• https://github.com/espressif/esp-idf/tree/master/examples/custom_bootloader/bootloader_override