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