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_x86_ab/BoardConfig.mk b/target/board/generic_x86_ab/BoardConfig.mk
index db4dacd..cc8ab70 100644
--- a/target/board/generic_x86_ab/BoardConfig.mk
+++ b/target/board/generic_x86_ab/BoardConfig.mk
@@ -14,17 +14,20 @@
 # limitations under the License.
 #
 
-include build/make/target/board/treble_common_32.mk
+include build/make/target/board/BoardConfigGsiCommon.mk
 
 TARGET_CPU_ABI := x86
 TARGET_ARCH := x86
 TARGET_ARCH_VARIANT := x86
 
+# Legacy GSI keeps 32 bits binder for 32 bits CPU Arch
+TARGET_USES_64_BIT_BINDER := false
+
+# Enable System-as-Root
+TARGET_NO_RECOVERY := true
+BOARD_BUILD_SYSTEM_ROOT_IMAGE := true
+
 # Set this to create /cache mount point for non-A/B devices that mounts /cache.
 # The partition size doesn't matter, just to make build pass.
 BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE := ext4
 BOARD_CACHEIMAGE_PARTITION_SIZE := 16777216
-
-# Enable A/B update
-TARGET_NO_RECOVERY := true
-BOARD_BUILD_SYSTEM_ROOT_IMAGE := true