firmware: search for third party or sysadmin supplied firmware updates

firmware: search for third party or sysadmin supplied firmware updates

We currently search /lib/firmware and /lib/firmware/`uname -r` for firmware
files for device drivers loaded by the currently running kernel. These are
often packaged by distributions as a subpackage of the kernel or as a
separate package containing firmware. But these files cannot easily be
updated by third parties or sysadmins independently of that package.

This patch causes udev to also look for firmware files in an "updates"
directory, which is almost identical in purpose to the module-init-tools
"updates" directories insomuch as local changes can go in here and will
take preference over firmware supplied by any distribution.