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.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
udev: ifdef all firmware special handling

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
keymap: Fix touchpad toggling on Lenovo IdeaPad U300s

IdeaPad U300s needs mapping 0xf1 to f21 just like Lenovo V480.

Signed-off-by: Martin Pitt <martinpitt@gnome.org>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>