Update Makefile

update to mpw-6b
diff --git a/Makefile b/Makefile
index 110aabf..fca06d4 100644
--- a/Makefile
+++ b/Makefile
@@ -28,7 +28,7 @@
 # Install lite version of caravel, (1): caravel-lite, (0): caravel
 CARAVEL_LITE?=1
 
-MPW_TAG ?= mpw-6a
+MPW_TAG ?= mpw-6b
 
 ifeq ($(CARAVEL_LITE),1)
 	CARAVEL_NAME := caravel-lite