bootcontrol: Update a check in set_active_boot_slot

set_active_boot_slot used to call into get_current_slot and assumed it
would get the currently active slot.

get_current_slot has been updated to return the slot number from which
we currently booted instead of it's earlier implementation which was to
return the active slot.

We now have a new function to return the current active slot which
set_active_boot_slot calls.

Change-Id: I191bb7c7f49ff75b972efb4a65e9bc9872c6e3d8