docs: add bootloaders build instructions

Several scripts have been developed to help user to generate bootloaders images. These scripts are hosted here: https://gitlab.com/baylibre/ti/android/bootloader/build-bootloaders

The documentation specifies instructions on how to use these scripts.

Co-authored-by: Julien Masson jmasson@baylibre.com

Signed-off-by: Safae Ouajih souajih@baylibre.com Signed-off-by: Julien Masson jmasson@baylibre.com