abootimg: Add init_boot image support

Quote from [1]:

"For devices launching with Android 13, the generic ramdisk is removed
from the boot image and placed in a separate init_boot image.
This change leaves the boot image with only the GKI kernel."

While at it, update wrong error handling message when vendor_boot
cannot be loaded.

[1]: https://source.android.com/docs/core/architecture/partitions/generic-boot



Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>