udhcpc: only background for MMU targets

package/busybox/config/networking/Config.in

@@ -1038,6 +1038,7 @@ source package/busybox/config/networking/udhcp/Config.in
 
 config BUSYBOX_IFUPDOWN_UDHCPC_CMD_OPTIONS
 	string "ifup udhcpc command line options"
+	default "-R" if BUSYBOX_NOMMU
 	default "-R -b"
 	depends on BUSYBOX_IFUPDOWN && BUSYBOX_UDHCPC
 	help