[OE-core] linux-firmware fetcher warnings

[Andre McCurdy]

The linux-firmware recipe is currently trying to fetch
iwlwifi-8000C-19.ucode which according to:

  https://bugs.launchpad.net/ubuntu/+source/linux-firmware/+bug/1579281

is an unreleased firmware image from a unofficial fork of the main
linux-firmware repo. The "iwlwifi-8000C-19.ucode" file isn't found in
the current master branch of the iwlwifi linux-firmware repo (and
doesn't seem to be there in the history either...).

The mystery firmware image was added to the linux-firmware recipe by:

  http://git.openembedded.org/openembedded-core/commit/?id=8b3d3ac84f787bf4ecccdcbcb97f2dac56acd45c

Is this firmware still required? If so, should SRC_URI be updated to
fetch it directly from downloads.yoctoproject.org?