boards: arm: stm32h573 disco kit can flash on external NOR

Configure the external NOR flash memory to be executable.
Change the board.cmake to add the external loader for
flashing (version that do not mass-erase the external flash).
The address of the external flash is not needed
when flashing a hex file.

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