buildman: Avoid using board as a variable

buildman: Avoid using board as a variable

We have a module called 'board'. Sometimes buildman uses 'brd' as an
instance variable but sometimes it uses 'board', which is confusing and
can mess with the module handling. Update the code to use 'brd'
consistently, making it easier for tools to determine when the module
is being referenced.

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