avbtool: Fix overallocation for Authentication Data block.

It was reported by Esun Kim that avbtool allocated too much space for
the authentication data block which resulted in vbmeta.img being
bigger than it needed to be. Better to only allocate the amount of
space of we need.

Bug: None
Test: Unit tests pass.
Test: Manually tested on UEFI based bootloader.

Change-Id: Id34ded854d0ee0d35f51122b9b20a3939a4cbc7f
3 files changed