platform/mulle: Add PLATFORM_HAS_LEDS define.

This is used by some of the Contiki examples to control conditional
compilation in order to add LED control to the application.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add #ifdefs to some defines.

Fixes compiler warnings when projects define their own settings on these.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>