ext: nordic: Correct CMakeLists.txt
ext: nordic: Correct CMakeLists.txt

This corrects two things:
- moves addition of the proper "system_*.c" file to sources, so that it
  is done only when CONFIG_HAS_NRFX is active (related CONFIG_SOC_* may
  be activated also for simulated targets which do not want to compile
  code with system initialization for the real SoC)
- removes redundant check of HAS_NRFX when adding "nrfx_glue.c"

Signed-off-by: Andrzej Głąbek <6788b8d556a8a447102338fc996c42180ecff44e@nordicsemi.no>

kraj/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,nashif/zephyr,explora26/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr