cpu/arm/k60: Avoid STOP when UART RX is active.

Added a check for all UART RAF flags.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Enable wake on UART.

This is necessary in order to be able to wake from STOP mode when
waiting for RX bytes.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Inhibit LLS when using SLIP.

UART RX can not wake CPU from LLS so we need to stay awake if we are
acting as a SLIP bridge.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add more LLS inhibitors to rf230

Added missing LLWU_INHIBIT_LLS() to init and transmit functions that
turn on the radio when it is turned off.

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