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

get_hashtree_descriptor() currently restricts HASHTREE descriptor to be
either in /vbmeta or in the same partition for dm-verity setup.

Also allows it to be from /boot partition because the top-level vbmeta
might be appended at /boot in legacy devices without /vbmeta.

Bug: 35880930
Test: test AVB on bullhead with top-level vbmeta being at /boot
Change-Id: I1ebd16a3c9f17bced6055146e8843b5918a737eb
(cherry picked from commit 37a0b318)