Fix flags argument passed to avbtool for GSI images

* avbtool expect extra flags to be passed with the --flags argument
  but currently we're missing an 's'.
  Luckily this isn't an issue as python argparse matches
  --flag as an alias to --flags ( python... )

Test: m vbmetaimage; check vbmeta header
Change-Id: I42d833c61c9b313dc067b1e519bae36395d09824
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index 1037eec..c67c4a3 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -33,13 +33,13 @@
 BOARD_USES_METADATA_PARTITION := true
 
 # Android Verified Boot (AVB):
-#   Set AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED (--flag 2) in
+#   Set AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED (--flags 2) in
 #   vbmeta.img to disable AVB verification.
 #
 # To disable AVB for GSI, use the vbmeta.img and the GSI together.
 # To enable AVB for GSI, include the GSI public key into the device-specific
 # vbmeta.img.
-BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --flag 2
+BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --flags 2
 
 # Enable chain partition for system.
 BOARD_AVB_SYSTEM_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem