buildman: Fix repeating board list with -l

Ensure that we don't print duplicate board names when -l is used.

Change-Id: I56adb138fc18f772ba61eba0fa194cdd7bc7efc6
Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>