Allow top-level vbmeta struct to be in 'boot' partition.

If there is no 'vbmeta' partition try to load the top-level vbmeta
struct from the end of 'boot' via a footer.

Two use-cases come to mind

 - bring-up when the partition table doesn't yet mention vbmeta; and
 - upgrades where it's not feasible to change the partition table

Bug: None
Test: New unit tests and all unit tests pass.
Change-Id: Id0c6c0f95ce157ffbeb0692d3c9547f49ab58640
5 files changed