Introduce support for -slot option for specifying slots,
and set_active, for changing the current slot.
Change-Id: Ib3b2a75491c0d0413534dd0c1d7bcb52555bba66
Check that the cache partition exists before trying to erase it.
Also clean up some of the C string handling and int booleans.
Bug: http://b/25375777
Change-Id: I1880e542b729f2026ab3a2943d4bee9d659b1eeb