fs_mgr_avb: allow top-level vbmeta struct to be in 'boot' partition

avb_ops->read_from_partition() should return
AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION if the requested partition does
not exist. Then libavb will try to load top-level vbmeta from /boot partition.

Two related changes in external/avb:

  https://android-review.googlesource.com/#/c/344907/
  https://android-review.googlesource.com/#/c/348731/

Bug: 35880930
Test: test AVB on bullhead with top-level vbmeta being at /boot
Change-Id: I80355ba5ae4d29a6f102714cd2f7c2550b238f27