build: Allow building disabled vbmeta images in signing process

If we sign builds out of tree there's no way to create a disabled vbmeta.img

Tests: Set
BOARD_AVB_ENABLE := false
BOARD_BUILD_DISABLED_VBMETAIMAGE := true
for taimen and follow https://source.android.com/devices/tech/ota/sign_builds
Result: signing will no longer fail due to missing vbmeta.img

Change-Id: If93182d0339d5fbc310373059716a56114d68d89