android: support boot header version 3 and 4

Add support for boot image version 3 and 4 in
android_image_get_dtb_img_addr().
Since the dtb is now included in vendor_boot image
instead of boot image, extract the dtb address from
vendor_boot image when a v3 or v4 is used.

Signed-off-by: Safae Ouajih <souajih@baylibre.com>