fix path to uevent helper

target/linux/kernel.config

@@ -29,6 +29,8 @@ CONFIG_MODULE_UNLOAD=y
 CONFIG_BINFMT_ELF=y
 CONFIG_BINFMT_SCRIPT=y
 CONFIG_HOTPLUG=y
+CONFIG_UEVENT_HELPER=y
+CONFIG_UEVENT_HELPER_PATH="/sbin/mdev"
 CONFIG_DEVTMPFS=y
 CONFIG_DEVTMPFS_MOUNT=y
 CONFIG_STANDALONE=y