buildman: Allow adjusting board config on the fly

Add a -a option to specify changes to the config before the build
commences. For example

   buildman -a ~CONFIG_CMDLINE

disables CONFIG_CMDLINE before doing the build.

This makes it easier to try things out as well as to write tests without
creating a new board or manually manging the .config file.

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