Add `full_boot` option to brillo_update_payload

Test: generate && apply an OTA with --full_boot
Bug: 171906589
Change-Id: I773e808ba5914e5fcc1c7be9095fdbee7d7ebd54

scripts/brillo_update_payload

diff --git a/scripts/brillo_update_payload b/scripts/brillo_update_payload
index 3bc87bd..4137dfd 100755
--- a/scripts/brillo_update_payload
+++ b/scripts/brillo_update_payload
@@ -649,7 +649,12 @@
     fi
     partition_names+="${part}"
     new_partitions+="${DST_PARTITIONS[${part}]}"
-    old_partitions+="${SRC_PARTITIONS[${part}]:-}"
+    if [ "${FLAGS_full_boot}" == "true" ] && [ "${part}" == "boot" ]; then
+      # Skip boot partition.
+      old_partitions+=""
+    else
+      old_partitions+="${SRC_PARTITIONS[${part}]:-}"
+    fi
     new_mapfiles+="${DST_PARTITIONS_MAP[${part}]:-}"
     old_mapfiles+="${SRC_PARTITIONS_MAP[${part}]:-}"
   done