Cleaned up slot logic

Updating old clean up slot logic change. Some devices don't know current
slot so we can't always just call get_current_slot()

Test: tested flashall on raven
Change-Id: Id655f1c1ee0b1ff09331f0d91d77412836d0af8d
diff --git a/fastboot/fastboot.h b/fastboot/fastboot.h
index c954487..6462a4f 100644
--- a/fastboot/fastboot.h
+++ b/fastboot/fastboot.h
@@ -80,7 +80,7 @@
     bool skip_secondary = false;
     bool force_flash = false;
 
-    std::string slot;
+    std::string slot_override;
     std::string current_slot;
     std::string secondary_slot;
     fastboot::FastBootDriver* fb;