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.

Bug: 29278988
Change-Id: Ia870e4be55617c0eefa4e1381287f8cd14affe7e