tests: build: Test build with MCUboot support

Add a new build variant that builds for Nordic nRF52832 with the new
MCUboot CONFIG_BOOTLOADER_MCUBOOT Kconfig variable set.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

tests/misc/test_build/testcase.yaml

@@ -16,3 +16,8 @@ tests:
     build_only: true
     extra_configs:
       - CONFIG_GDB_SERVER=y
+  test_mcuboot_bootloader:
+    build_only: true
+    platform_whitelist: nrf51_pca10028 nrf52_pca10040
+    extra_configs:
+      - CONFIG_BOOTLOADER_MCUBOOT=y