Reenable avb for mainline

Partial rollback of Id5d114bde8b93800fe52c791ccc47cdffd86f382.

It is likely mainline also wants some of the BOARD_AVB_SYSTEM.. flags,
but I'm not entirely sure which ones.

Test: run build_mixed with this patch
Change-Id: I6ef3c01f6c74e6d93cf798b26b8c7a4785bbefb4
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index a52bb80..73af020 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -18,15 +18,12 @@
 BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE := 67108864
 
 # Android Verified Boot (AVB):
-#   1) Sets BOARD_AVB_ENABLE to sign the GSI image.
-#   2) Sets AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED (--flag 2) in
-#      vbmeta.img to disable AVB verification.
+#   Set AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED (--flag 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_ENABLE := true
-BOARD_AVB_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
 BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --flag 2
 
 # Enable chain partition for system.
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index 1cbe2e0..ec3c74f 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -23,3 +23,6 @@
 
 # Audio: must using XML format for Treblized devices
 USE_XML_AUDIO_POLICY_CONF := 1
+
+BOARD_AVB_ENABLE := true
+BOARD_AVB_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)