This adds the SPDX-License-Identifier license headers to the majority of
our source files to make it clearer exactly which license files are under.

The bulk of the files are under GPL v2.0 with one found to be under V2.0
or later, some under MIT and some have dual license. There are some files
which are potentially harder to classify where we've imported upstream code
and those can be handled specifically in later commits.

The COPYING file is replaced with LICENSE.X files which contain the full
license texts.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>