samples: sysbuild: with mcu_boot on the stm32h7rs targets

Run the sample to execute in place on the external flash of the
stm32h7s78_dk or nucleo_h7s3l8 boards.
with_boot is build and linked for the external flash
Download the mcuboot zephyr.bin in internal memory
Download the with_mcuboot zephyr.signed.bin in external memory

Signed-off-by: Francois Ramu <francois.ramu@st.com>