Handle virtual a/b

Handle virtual a/b

This commit adds support for virtual a/b. It is assumed
that super contains only one set of partitions instead
of two slots. A dm-snapshot is created during upgrade
which will be treated as a next partition and change
will be applied to it. After booting to fota and
successful rw update the snapshot will be merged to
an origin partition.

Change-Id: Idebaffb0ea7b77e3cede0d40f99459fed83482b7