boards: st: fix sysbuild multi image flash

Problem:

When flashing a multi-image project with STLink through sysbuild,
the flash utility is told to erase the whole flash between each
single image flash.

Resulting in a partial flash where only the last image is effectively
stored on flash...

Correction:

A `west flash` must not implicitly perform a mass erase on its own.

If a flash erase is required, the option has to be passed manually.

The problem is discussed in the following issue:
zephyrproject-rtos/zephyr#69582

Due to CI tests errors, the correction is not applied on
eval board `b_u585i_iot02a`.

See following issue:
zephyrproject-rtos/zephyr#75164

Signed-off-by: Alex Fabre <alex.fabre@rtone.fr>