Allow specifying vbmeta/parts via fstab

We want to move all information required for first stage mount into
the first stage ramdisk instead of using device tree.  A previous
change allowed reading the fstab from the first stage ramdisk, but
'vbmeta/parts' was still in device tree.  This change allows
specifying the required vbmeta/parts information in the fstab.

The new syntax is that instead of simply specifying 'avb' as an fs_mgr
option, a partition may specify 'avb=<vbmeta partition name>' and that
vbmeta partition will also be initialized by first stage init before
attempting to mount any devices.

Bug: 117933812
Test: boot crosshatch without device tree fstab
Change-Id: Ida1a6da988c10d364b3ccdaa6c5d63e5264d1b27