Add BOARD_MOVE_GSI_AVB_KEYS_TO_VENDOR_BOOT

BOARD_MOVE_GSI_AVB_KEYS_TO_VENDOR_BOOT controls whether GSI AVB keys are built
to vendor_boot. On devices without a dedicated recovery partition, GSI AVB
keys used to be built in boot. They are device-specific, so they should be
moved to vendor_boot.

Test: pass
Bug: 156098440
Change-Id: I0a5eaa1b39e88fcca9837c7aa3f475be47d5b0f2