common: board_f: cosmetic use __weak for leds

First of all this looks a lot better, but it also
prevents a gcc warning (W=1), that the weak function
has no previous prototype.

cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Simon Glass <sjg@chromium.org>