treble_common.mk: add the vbmeta.img to disable verification

AVB should be disabled for Treble VTS. Otherwise, OEM devices cannot boot
the generic system.img provided by Google.

Bug: 64055917
Test: Compiles and boot a device with the vbmeta.img
Change-Id: I0eaae7beebc8b901d0b4766c2cdc34a27ad02b59
2 files changed