Add "entropy" experiment flag

For experiments to improve compression efficiency of entropy coding.

Change-Id: Idf30dc554bd7eea3a79d21d18515c3e6b8f64b26
diff --git a/configure b/configure
index 97366e4..ed1d048 100755
--- a/configure
+++ b/configure
@@ -284,6 +284,7 @@
     ext_partition
     ext_tile
     obmc
+    entropy
 "
 CONFIG_LIST="
     dependency_tracking