Fix GKI 2.0 signing in the release process

In the release signing process, it uses --signing_helper_with_files
for avbtool. This can be done by passing
--gki_signing_signature_args="--signing_helper_with_files=/path/to/helper.sh"
for mkbootimg.

However, this only works when there is a default value of
"gki_signing_signature_args" in the misc_info.txt, to be appended
with more extra args. Adding a default value "--prop foo:bar" to make
it work.

Note: also move the BOARD_GKI_* signing config to
BoardConfigGsiCommon.mk, so we don't have to duplicate the settings
for each architecture.

Bug: 178559811
Bug: 177862434
Test: `make bootimage`
      `unpack_bootimg --boot_img $OUT/boot.img --out unpack`
      `avbtool info_image --image ./unpack/boot_signature`
Change-Id: I8bd8ad3acf324931b47d45fd30bc590206b1927e
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index c577870..95ba1d0 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -30,6 +30,14 @@
 # the devices with metadata parition
 BOARD_USES_METADATA_PARTITION := true
 
+# Enable GKI 2.0 signing.
+BOARD_GKI_SIGNING_KEY_PATH := build/make/target/product/gsi/testkey_rsa2048.pem
+BOARD_GKI_SIGNING_ALGORITHM := SHA256_RSA2048
+# The following is needed to allow release signing process appends more extra
+# args, e.g., passing --signing_helper_with_files from mkbootimg to avbtool.
+# See b/178559811 for more details.
+BOARD_GKI_SIGNING_SIGNATURE_ARGS := --prop foo:bar
+
 # Android Verified Boot (AVB):
 #   Set the rollback index to zero, to prevent the device bootloader from
 #   updating the last seen rollback index in the tamper-evident storage.
diff --git a/target/board/generic_arm64/BoardConfig.mk b/target/board/generic_arm64/BoardConfig.mk
index 1229327..423faf9 100644
--- a/target/board/generic_arm64/BoardConfig.mk
+++ b/target/board/generic_arm64/BoardConfig.mk
@@ -77,10 +77,6 @@
 BOARD_BOOT_HEADER_VERSION := 4
 BOARD_MKBOOTIMG_ARGS += --header_version $(BOARD_BOOT_HEADER_VERSION)
 
-# Enable GKI 2.0 signing.
-BOARD_GKI_SIGNING_KEY_PATH := build/make/target/product/gsi/testkey_rsa2048.pem
-BOARD_GKI_SIGNING_ALGORITHM := SHA256_RSA2048
-
 BOARD_KERNEL_BINARIES := \
     kernel-4.19-gz \
     kernel-5.4 kernel-5.4-gz kernel-5.4-lz4 \