AVB: support signing vendor.img

Uses avbtool to sign vendor.img if BOARD_AVB_ENABLE is set.
It also allows appending additional arguments to avbtool via
BOARD_AVB_VENDOR_ADD_HASHTREE_FOOTER_ARGS.

e.g.,
  BOARD_AVB_ENABLE := true
  BOARD_AVB_VENDOR_ADD_HASHTREE_FOOTER_ARGS := --generate_fec

Bug: 35415839
Test: "make" with the above variables and use avbtool to check vbmeta is
      appended to vendor.img
Test: "make dist" with the above variables

Change-Id: I8ada38dff3def6d34613e77c67944def8a49f464
Merged-In: I8ada38dff3def6d34613e77c67944def8a49f464
(cherry picked from commit 8ee4a3db)