add API to get hash descriptor

The GKI verification VTS test will need to examine the boot partition's
hash descriptor, so add support to access this descriptor.

Bug: 148800209
Test: atest AvbTest#Boot
Change-Id: I92e32f61a265671ae0940c44147391f73776e66a
Merged-In: I92e32f61a265671ae0940c44147391f73776e66a
5 files changed