docbook-xml-dtd4: clean up LICENSE situation

docbook-xml-dtd4: clean up LICENSE situation

Don't use the common 'OASIS' license, as this isn't a SPDX license and
is in fact specifically the DocBook XML v4 license text.

Instead, use NO_GENERIC_LICENSE to ship a custom license file. We still
need to ship a copy of it in-tree because the license code needs a bare
copy of the license text.  Reference the in-tree license text in the
license checksum to show that we've verified they match.

Also, clean up some comments.

Thanks to hongxu.jia@eng.windriver.com for a previous iteration of this.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>