Enable AVB signing on GSI

In current GSI builds, AVB signing is not allowed.
We should allow signing GSI image, while building a vbmeta.img with
AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED (flag 2).

When need to disable AVB, use the vbmeta.img above together with the
GSI. When need enable AVB on GSI, include the GSI public key into the
device-specific vbmeta.img. Note that GSI is a chain partition so the
keys used to sign GSI and vbmeta.img are different.

Bug: 112293933
Test: build aosp_arm64-userdebug, checks flag 2 is set in vbmeta.img
      and there is AVB HASHTREE in system.img
      `avbtool info_image --image $OUT/system.img`
      `avbtool info_image --image $OUT/vbmeta.img`

Change-Id: If1eff317621882994e7dcbf37cd35f5bfa16b69a
diff --git a/target/board/BoardConfigEmuCommon.mk b/target/board/BoardConfigEmuCommon.mk
index ca2176c..a1d5cde 100644
--- a/target/board/BoardConfigEmuCommon.mk
+++ b/target/board/BoardConfigEmuCommon.mk
@@ -28,8 +28,9 @@
 TARGET_COPY_OUT_VENDOR := vendor
 
 # ~100 MB vendor image. Please adjust system image / vendor image sizes
-# when finalizing them.
-BOARD_VENDORIMAGE_PARTITION_SIZE := 100000000
+# when finalizing them. The partition size needs to be a multiple of image
+# block size: 4096.
+BOARD_VENDORIMAGE_PARTITION_SIZE := 100003840
 BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE := ext4
 BOARD_FLASH_BLOCK_SIZE := 512
 DEVICE_MATRIX_FILE   := device/generic/goldfish/compatibility_matrix.xml