boot: android: Provide vendor_bootimg_addr in boot_get_fdt()

boot: android: Provide vendor_bootimg_addr in boot_get_fdt()

When calling android_image_get_dtb_by_index() using boot image v3+,
we should also pass the vendor_boot ramdisk address.

Use get_avendor_bootimg_addr() to do so.

Notes: on boot image v2, this is harmless since get_avendor_bootimg_addr()
       returns -1.
       for legacy implementations that don't have CMD_ABOOTIMG, add a weak
       implementation to avoid linking errors.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Guillaume La Roque <glaroque@baylibre.com>
Tested-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>