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/engine.cpp b/fastboot/engine.cpp
index 44796d7..d4be63b 100644
--- a/fastboot/engine.cpp
+++ b/fastboot/engine.cpp
@@ -130,6 +130,13 @@
     return a;
 }
 
+void fb_set_active(const char *slot)
+{
+    Action *a;
+    a = queue_action(OP_COMMAND, "set_active:%s", slot);
+    a->msg = mkmsg("Setting current slot to '%s'", slot);
+}
+
 void fb_queue_erase(const char *ptn)
 {
     Action *a;