buildman: Just display a single line for missing maintainers

At present we get multiple lines of output when a board has no MAINTAINERS
entry:

   WARNING: no status info for 'bananapi-m2-pro'
   WARNING: no maintainers for 'bananapi-m2-pro'

Suppress the 'status' one since it is implied by the other.

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