Fastboot changes to support A/B partitioning

Introduce support for -slot option for specifying slots,
and set_active, for changing the current slot.

Change-Id: Ib3b2a75491c0d0413534dd0c1d7bcb52555bba66
(cherry picked from commit b7bd4ae529148e7f4d830d98ea0a75bdc5d1ac17)
3 files changed