Allow vendors to add custom vbmeta partitions

Currently we only have vbmeta_system and vbmeta_vendor. Add
BOARD_AVB_VBMETA_CUSTOM_PARTITIONS , which allows OEMs to define
their own vbmeta_*.img partitions. example: aosp/2381823

Bug: 263568805
Change-Id: Id671e2c3aee9ada90256381cce432927df03169b
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 2f05d44..9919029 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -1503,12 +1503,14 @@
 
   custom_partitions = OPTIONS.info_dict.get(
       "avb_custom_images_partition_list", "").strip().split()
+  custom_avb_partitions = ["vbmeta_" + part for part in OPTIONS.info_dict.get("avb_custom_vbmeta_images_partition_list", "").strip().split()]
 
   for partition, path in partitions.items():
     if partition not in needed_partitions:
       continue
     assert (partition in AVB_PARTITIONS or
             partition in AVB_VBMETA_PARTITIONS or
+            partition in custom_avb_partitions or
             partition in custom_partitions), \
         'Unknown partition: {}'.format(partition)
     assert os.path.exists(path), \