kconfig: Improve USE_CODE_PARTITION prompt and help string
kconfig: Improve USE_CODE_PARTITION prompt and help string

The prompt and help string for USE_CODE_PARTITION were too terse and
didn't make it clear that it's related to devicetree, which confused me.
Spell things out in more detail.

Unless the meaning of a symbol is completely obvious from context, aim
for at least a few sentences of help text. Think about what would be
confusing for someone coming at it without much context.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr