kconfig: Make FLASH_LOAD_OFFSET/SIZE non-configurable when DT is used
kconfig: Make FLASH_LOAD_OFFSET/SIZE non-configurable when DT is used

Having FLASH_LOAD_OFFSET and FLASH_LOAD_SIZE always configurable froze
their values at 0 when BOOTLOADER_MCUBOOT was enabled in menuconfig,
when instead the values from /chosen/zephyr,code-partition in devicetree
should be used. BOOTLOADER_MCUBOOT selects USE_CODE_PARTITION, which is
a flag to use the devicetree information.

To fix it, only make FLASH_LOAD_OFFSET and FLASH_LOAD_SIZE configurable
when USE_CODE_PARTITION is disabled. It looks like no configuration
files set them at the moment.

See the added documentation in
https://github.com/zephyrproject-rtos/zephyr/pull/20722 for an
explanation of why this happens. This bit novalisek in
https://github.com/zephyrproject-rtos/zephyr/issues/20673.

Fixes: #20673

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

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