Reenable avb for mainline

Partial rollback of Id5d114bde8b93800fe52c791ccc47cdffd86f382.

It is likely mainline also wants some of the BOARD_AVB_SYSTEM.. flags,
but I'm not entirely sure which ones.

Test: run build_mixed with this patch
Change-Id: I6ef3c01f6c74e6d93cf798b26b8c7a4785bbefb4