tests: external_flash: sysbuild: Enable empty netcore

Updated configuration to enable empty netcore core image.

Signed-off-by: Grzegorz Chwierut <[email protected]>

tests/modules/mcuboot/external_flash/Kconfig.sysbuild

@@ -7,4 +7,18 @@
 config PM_EXTERNAL_FLASH_MCUBOOT_SECONDARY
 	default y if BOARD_NRF5340DK_NRF5340_CPUAPP || BOARD_NRF52840DK_NRF52840
 
+choice MCUBOOT_MODE
+	default MCUBOOT_MODE_OVERWRITE_ONLY if SOC_SERIES_NRF53X
+endchoice
+
+choice NETCORE
+	default NETCORE_EMPTY if SOC_NRF5340_CPUAPP
+endchoice
+
+config SECURE_BOOT_NETCORE
+	default y if SOC_NRF5340_CPUAPP
+
+config NETCORE_APP_UPDATE
+	default y if SOC_NRF5340_CPUAPP
+
 source "${ZEPHYR_BASE}/share/sysbuild/Kconfig"