fix build mikrotik for beta

builder.sh

@@ -100,7 +100,7 @@ do
     echo "#######################################"
     echo Building $ARCHITECTURE
     if [ "$ARCHITECTURE" = "ar71xx-mikrotik" ]; then
-        if [ "$BRANCH" = "beta" ]; then
+        if [[ "$BRANCH" == *"beta" ]]; then
             echo "Allowing to bild ar71xx-mikrotik since we build for beta"
 	else
 	    echo "Skipping build for ar71xx-mikrotik since we do not build for beta"