update_verifier: Support androidboot.veritymode being empty or 'disabled'.

Bootloaders using libavb will set androidboot.veritymode=disabled if
the "disable dm-verity" flag has been set. Additionally if the
"disable verification" flag is set androidboot.veritymode will not be
set at all. Handle both cases.

Without this fix we'll end up in a bootloop.

Test: Manually tested on a device using AVB.
Bug: 64315394
Change-Id: I8310849e347248f4a96158838310f688ecef4211