samples: sysbuild: with_mcuboot: Replace overlay with new board variant

Replace overlay for stm32h573i_dk with new board variant designed to
run in external flash.

Signed-off-by: Tim Pambor <[email protected]>

Author: tpambor

samples/sysbuild/with_mcuboot/boards/stm32h573i_dk.overlay

@@ -17,7 +17,7 @@ tests:
       - nucleo_h7s3l8
       - nucleo_u385rg_q
       - stm32h7s78_dk
-      - stm32h573i_dk
+      - stm32h573i_dk/stm32h573xx/ext_flash_app
     integration_platforms:
       - nrf52840dk/nrf52840
       - esp32_devkitc/esp32/procpu