Add rd_debug flag

rd_debug is a debug tool aim at finding discrepancy between rate-distortion
loop and bitstream packing.

Change-Id: I751c4121516c5e6368668229c77778880a9dcb9d
diff --git a/configure b/configure
index 3258ee3..9505b05 100755
--- a/configure
+++ b/configure
@@ -294,6 +294,7 @@
     tile_groups
     ec_adapt
     simp_mv_pred
+    rd_debug
 "
 CONFIG_LIST="
     dependency_tracking