Refactor GSI BoardConfig.mk

The patch modifies the Legacy GSI to use BoardConfigGsiCommon.mk
instead of board config treble-common*.mk, and delete these
treble-common*.mk files.

After the patch, the GSI common board configs are in single file
BoardConfigGsiCommon.mk

The patch also includes:

- Moving `TARGET_COPY_OUT_VENDOR := vendor` to
  BoardConfigGsiCommon.mk, because GSI always contains no vendor
  contents.
- Configure the binder interface bitness explicitly.
- Unify the structure of each BoardConfig.mk.

Bug: 122282239
Test: compare the out folder between with and without the patch
Test: output files are all the same
Change-Id: Ie5901446920dda317dea848e6d5a7bbcf10a32cf
diff --git a/target/board/generic_arm64_ab/BoardConfig.mk b/target/board/generic_arm64_ab/BoardConfig.mk
index 88b90a8..073eccb 100644
--- a/target/board/generic_arm64_ab/BoardConfig.mk
+++ b/target/board/generic_arm64_ab/BoardConfig.mk
@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 
-include build/make/target/board/treble_common_64.mk
+include build/make/target/board/BoardConfigGsiCommon.mk
 
 TARGET_ARCH := arm64
 TARGET_ARCH_VARIANT := armv8-a
@@ -28,7 +28,7 @@
 TARGET_2ND_CPU_ABI2 := armeabi
 TARGET_2ND_CPU_VARIANT := generic
 
-# Enable A/B update
+# Enable System-as-Root
 TARGET_NO_RECOVERY := true
 BOARD_BUILD_SYSTEM_ROOT_IMAGE := true