app: aboot: A/B support for Automotive slot lookup and boot

Adding A/B support for automotive.  Enable slot lookup from
partitions.  Add functionality for fastboot commands and updates
for getvar.
This is a variant of change from
I51628bd4f9e16b053461b8e922a31cbdb521f535 from mobile

Change-Id: I0500b17d72f4a8e0c34c3bb7d8b6e2423b2e1965