linux-yocto/5.13: add devupstream support

We are always getting questions about building -stable, or mainline
master. Rather than introducing a separate set of recipes, we can
facilitate this sort of usecase by using the existing devupstream
bbclass support.

To build an unpatched or otherwise modifed -stable of 5.13, simply
set the linux-yocto preferred provider to:

  linux-yocto-upstream

and the PREFERRED_VERSION_linux-yocto-upstream to one with support.

And your wish will be granted, as the build will use v5.13/base
which matches the main linux-yocto version, simply with no extra
changes applied.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>