platform/mulle: Add button-sensor.c stub to Makefile.

Required for building rpl-border-router.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Only define UIP_CONF_TCP if unset

contiki-conf.h should only provide a default value when the project
has no local definition.

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