buildman: Separate out display of warnings and errors

Some boards unfortunately build with warnings and it is useful to be able
to easily distinguish the warnings from the errors.

Use a simple pattern match to categorise gcc output into warnings and
errors, and display each separately. New warnings are shown in magenta (with
a w+ prefix) and fixed warnings are shown in yellow with a w- prefix.

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