mkfirmware.sh: Keep the original rockdev/

Other stages in build.sh might generate something in rockdev/.

Change-Id: I37349b9a8cb17e2ac59dfdeb76fdb328e5ed0aa1
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>

common/mkfirmware.sh

@@ -56,9 +56,7 @@ fatal() {
     exit -1
 }
 
-# Clean all
 mkdir -p $ROCKDEV
-rm -rf $ROCKDEV/*
 
 # Require buildroot host tools to do image packing.
 if [ ! -d "$TARGET_OUTPUT_DIR" ]; then