patman: additionally honor a local .patman config file

This enables versioning a project specific patman configuration file.
It also makes it possible to declare the project name, which is not a
useful thing to do in $HOME/.patman.  A new test is added, along
updated documentation.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Reviewed-by: Simon Glass <sjg@chromium.org>