build_helper_configs: Set MCUboot dummy keys for stm32u5xxx

Add a workaround to set MCUboot dummy keys as default signing keys via
command line on b_u585i_iot02a platform.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I208255d7fb1c96a5966fa0d2789f44f5e59d72c0
2 files changed