busybox: remove config dependency on busybox-source

busybox-source target doesn't need to depend on the config file.
Patch by Simon Pasch.

package/busybox/busybox.mk

@@ -125,7 +125,7 @@ endif
 
 busybox: uclibc $(TARGET_DIR)/bin/busybox
 
-busybox-source: $(DL_DIR)/$(BUSYBOX_SOURCE) $(BUSYBOX_CONFIG_FILE)
+busybox-source: $(DL_DIR)/$(BUSYBOX_SOURCE)
 
 busybox-unpacked: $(BUSYBOX_DIR)/.unpacked