udev: make firmware loading optional and disable by default

Distros that whish to support old kernels should set
  --with-firmware-dirs="/usr/lib/firmware/updates:/usr/lib/firmware"
to retain the old behaviour.

rules/50-firmware.rules

@@ -0,0 +1,3 @@
+# do not edit this file, it will be overwritten on update
+
+SUBSYSTEM=="firmware", ACTION=="add", RUN{builtin}="firmware"

rules/50-udev-default.rules

@@ -64,5 +64,4 @@ SUBSYSTEM=="virtio-ports", KERNEL=="vport*", ATTR{name}=="?*", SYMLINK+="virtio-
 
 SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", IMPORT{builtin}="usb_id", IMPORT{builtin}="hwdb --subsystem=usb"
 SUBSYSTEM=="input", ENV{ID_INPUT}=="", IMPORT{builtin}="input_id"
-SUBSYSTEM=="firmware", ACTION=="add", IMPORT{builtin}="firmware"
 ENV{MODALIAS}!="", IMPORT{builtin}="hwdb --subsystem=$env{SUBSYSTEM}"