west: sign: rimage: drop default -c option when the user provides one

sign.py has an internal and indirect way to compute a default -c signing
schema option and pass it to rimage. It is built by appending
`$platform.toml` to whatever `rimage/config/` location was
found. Defaults are very convenient but in this case this computed -c
option can conflict with an explicit -c option spelled out by the user.

Scan for any explicit -c coming directly from the user. If any found,
ignore the default we computed. This is what is already being done for -k.

The precedence across rimage parameters coming from different places is
too complicated. Not passing multiple -c options simplifies the logic a
little bit.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>