cpu/arm/k60: Dead code removal during linking.

Add -ffunction-sections and -fdata-sections to CFLAGS in order to put
each function in its own section and then remove unused sections during
linking.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Share debug uart functions with other arm ports.

Removed LW printf and modified API to match stm32w port.

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