linker: Minor refactor of the APP_SMEM_SECTION macro
linker: Minor refactor of the APP_SMEM_SECTION macro

Minor refactoring and commenting of the _SECTION infrastructure in
preparation for future improvements.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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