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
diff --git a/fastboot/fastboot.h b/fastboot/fastboot.h
index 9e33531..784ec5c 100644
--- a/fastboot/fastboot.h
+++ b/fastboot/fastboot.h
@@ -64,6 +64,7 @@
 void fb_queue_notice(const char *notice);
 void fb_queue_wait_for_disconnect(void);
 int fb_execute_queue(usb_handle *usb);
+void fb_set_active(const char *slot);
 
 /* util stuff */
 double now();