buildman: Add the option to download toolchains from kernel.org

The site at https://www.kernel.org/pub/tools/crosstool/

 is a convenient
repository of toolchains which can be used for U-Boot. Add a feature to
download and install a toolchain for a selected architecture automatically.

It isn't clear how long this site will stay in the current place and
format, but we should be able to rely on bug reports if it changes.

Suggested-by: Marek Vašut <marex@denx.de>
Suggested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Simon Glass <sjg@chromium.org>