cmake: update common Zephyr build settings to use `zephyr_get()`

Fixes: #40389

This commit aligns the use of the following Zephyr build variables:
- CONF_FILE
- DTC_OVERLAY_FILE
- OVERLAY_CONFIG
- WEST_DIR
- ZEPHYR_BOARD_ALIASES
- ZEPHYR_EXTRA_MODULES
- ZEPHYR_MODULES

so that they can be set in same manner and with same order of
preference.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>