Support appending vbmeta digest to id/fingerprint

Background in http://go/compatible-build-fingerprint. To uniquely
identify the mixed build, we plan to append the unique vbmeta digest
to ro.build.id.

If BOARD_USE_VBMETA_DIGTEST_IN_FINGERPRINT is true, the build system
will not set ro.build.id. Instead, init will set it at runtime, by
appending the digest to the legacy build id.

Bug: 186786987
Test: build and boot a device with new build id
Change-Id: Idea57df599bfd6eede760671e2555541f7dc3f21