Add Fastboot support for flashing secondary images

Fastboot update and flashall will now flash secondary
images if provided, and if the --slot flag is not set
to 'all'. Also added flash-primary to preserve the
previous behavior, and flash-secondary to just flash
the secondary images.

Fixes from: I5bd2de1c1e0e6224a195b566f7dcbe383555a80a

Bug: 29278988
Change-Id: Ia870e4be55617c0eefa4e1381287f8cd14affe7e
(cherry-picked from commit 6c98509c)