Introduce support for -slot option for specifying slots,
and set_active, for changing the current slot.
Change-Id: Ib3b2a75491c0d0413534dd0c1d7bcb52555bba66
(cherry picked from commit b7bd4ae529)
This brings us all the fastboot changes, plus a change we need from libbase,
and then reverts part of a libziparchive cleanup and fixes dependencies on
new changes to the build system.
Bug: http://b/25375777
Change-Id: I813464da95dc6c81b0a1dc7145152f89bb2d14cd