buildman: Add helper functions for updating .config files

At present the only straightforward way to write tests that need a
slightly different configuration is to create a new board with its own
configuration. This is cumbersome.

It would be useful if buildman could adjust the configuration of a build
on the fly. In preparation for this, add a utility library which can
modify a .config file according to various parameters passed to it.

Signed-off-by: Simon Glass <sjg@chromium.org>