buildman: Show the list of boards in magenta

buildman: Show the list of boards in magenta

It is quite hard to see the list of board for each error line since the
colour is the same as the actual error line. Show the board list in
magenta so that it is easier to distinguish them.

There is no point in checking the colour of the overall line, since there
are now multiple colours. So drop those tests.

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