cmake: LD: Add comment explaining the -P flag
cmake: LD: Add comment explaining the -P flag

Add a comment that explains what the -P flag does. The explanatory
comment is useful when you want to find the flag so you can comment it
out while debugging the link stage.

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

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