Prevent CPU from using cortexm_init() when softdevice is used for NRF52 devices
as the softdevice already do some obscur inits
Also ensure the softdevice pkg will be used by hacking Makefile due to RIOT's
build system limitations (will be fix later with the new build system)
This function is supposed to be used for putting the CPU into
sleep mode for short amounts of time (e.g. in typical polling
loops used in periph drivers).
