scripts: runners: stm32cubeprogrammer: Review f/w selection algorithm

Aim is to avoid 2 issues:
- Requesting --download-address to flash hex files
- Flashing hex files (non signed) on N6 which works but doesn't allow
persistent firmware

Hence this changes binds --download-address with usage of .bin files.
If --download-address was not provided, default to hex flashing.

File existence done with isfile() will ensure the required file
is available before flashing and report an error if this is not the
case.
Note: In specific N6 case, we're verifying zephyr.signed.bin is
available instead of zephyr.bin. This is ensure since self.cfg.bin_file
matches runners_yaml_props_target configuration (set to
zephyr.signed.bin on STM32N6).

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>