platform/mulle: Add extern "C" to all headers.

Added #ifdef __cplusplus and extern "C" to all platform/mulle headers
minus one unused legacy AVR header for the radio.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add extern "C" to all headers.

Added #ifdef __cplusplus and extern "C" to all cpu/arm/k60 headers
minus the Freescale-provided register map headers.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add missing include guard.

Added missing include guard in core-clocks.h

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