patman: Add the concept of multiple projects

There are cases that we want to support different settings (or maybe
even different aliases) for different projects.  Add support for this
by:
* Adding detection for two big projects: U-Boot and Linux.
* Adding default settings for Linux (U-Boot is already good with the
  standard patman defaults).
* Extend the new "settings" feature in .patman to specify per-project
  settings.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>