android: boot: ramdisk: support vendor ramdisk

Version 3 and 4 of boot image header introduced
vendor boot ramdisk: Please check include/android_image.h
for details.

The ramdisk is now split into a generic ramdisk in boot image
and a vendor ramdisk in vendor boot image.

Support the new vendor ramdisk.

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