doc: android: add documentation for v3, v4 boot image header

Update the Android documentation to describe version 3 and 4 of boot
image header.

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
1 file changed