samples: sysbuild: with_mcuboot: stm32h7s78_dk: Use ext_flash_app variant

Now that ext_flash_app variant is available, it should be used for
mcuboot related apps on this board.

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

samples/sysbuild/with_mcuboot/sample.yaml

@@ -16,7 +16,7 @@ tests:
       - esp32c6_devkitc/esp32c6/hpcore
       - nucleo_h7s3l8
       - nucleo_u385rg_q
-      - stm32h7s78_dk
+      - stm32h7s78_dk/stm32h7s7xx/ext_flash_app
       - stm32h573i_dk
     integration_platforms:
       - nrf52840dk/nrf52840