Add a way to link against different driver names.

The existing linker path hard-coded "-lRSDriver" into the command line, but
this won't work for partner drivers that have a different implementation
loaded. In order to still properly handle use of the CPU driver, this needs
to change depending on whether we actually loaded an OVERRIDE_RS_DRIVER or
not.

Change-Id: I0c4a4f12f5db819b234952bc8f364ac6300f147b
diff --git a/rsContext.cpp b/rsContext.cpp
index 0bf9d9d..29b4036 100644
--- a/rsContext.cpp
+++ b/rsContext.cpp
@@ -517,6 +517,7 @@
     }
 
     mHasSurface = false;
+    mDriverName = NULL;
 
     timerInit();
     timerSet(RS_TIMER_INTERNAL);