Now that we are moving away from boilerplate code towards SPDX license
tags in individual file headers, we should have a short note about what
these tags mean. Also, until now, there was effectively no explicit
information that barebox is licensed under GPLv2 (only). This could have
been interpolated from the existence of a GPLv2 text in ./COPYING (and
from most of the individual file headers… have fun reading all of them),
but a warranty and copyright notice was still missing. Add those as a
catch-all for the project to ease application of GPLv2 article 0:

      0. This License applies to any program or other work which contains
    a notice placed by the copyright holder saying it may be distributed
    under the terms of this General Public License. […]

and article 1:

      1. You may copy and distribute verbatim copies of the Program's
    source code as you receive it, in any medium, provided that you
    conspicuously and appropriately publish on each copy an appropriate
    copyright notice and disclaimer of warranty; […]

Since barebox goes back to U-Boot, I tried to find explicit project-wide
copyright lines, but the only one I could find was the one from U-Boot's
old README in commit 7e149c897b95b40752. If there are additional
copyright holders, those are probably only present in the respective
file headers.

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>