doc: relocate/rename Android README and add BCB overview

Rename:
 - doc/{README.avb2 => android/avb2.txt}
 - doc/{README.android-fastboot => android/fastboot.txt}

Add a new file documenting the 'bcb' command:
 - doc/android/bcb.txt

The new directory structure has been reviewed by Simon in
https://patchwork.ozlabs.org/patch/1101107/#2176031

 .

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>