zephyr bugfix: mcuboot doesn't link into code-partition

Select the code partition as the target for link into.
This patch together with
https://github.com/zephyrproject-rtos/zephyr/pull/14620
fixes the issue which was imposed while zephyr moves
DTC before Kconfig processing in its build process.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 4e75cef..b44ca68 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -12,6 +12,7 @@
 	default y
 	bool
 	select MPU_ALLOW_FLASH_WRITE if ARM_MPU
+	select USE_CODE_PARTITION if HAS_FLASH_LOAD_OFFSET
 
 if BOARD_HAS_NRF5_BOOTLOADER