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>
diff --git a/include/image.h b/include/image.h
index acffd17..c5b288f 100644
--- a/include/image.h
+++ b/include/image.h
@@ -1971,6 +1971,13 @@
 ulong get_abootimg_addr(void);
 
 /**
+ * get_ainit_bootimg_addr() - Get Android init boot image address
+ *
+ * Return: Android init boot image address
+ */
+ulong get_ainit_bootimg_addr(void);
+
+/**
  * get_avendor_bootimg_addr() - Get Android vendor boot image address
  *
  * Return: Android vendor boot image address