sign.py: stop ignoring the -- -c foo.toml option passed to rimage

The -c option points rimage at its main and mandatory configuration
file ("signing schema"). The -c option passed by sign.py to rimage comes
from _two_ different places:

A. From the command line, example:

   west sign -t rimage -- OTHER_ARGS_FOR_RIMAGE -c foo.toml

However passing -- -c signing_schema.toml on the west sign command line
has always been optional because:

B. west sign systematically adds another `-c bar.toml` option. The name
   'bar' is found in the CMakeCache. Right now 'bar' comes from
   a product specific `board.cmake` file.

There were two problems fixed by this commit:

1. The -c option from the command line was passed _first_ but the last
   -c wins with rimage. The command line should have precedence.

2. The "last -c wins" behavior is not documented/official, it's an
   rimage implementation deteail.

To fix both, simply scan the command line for a '-c' option. If any is
found then it takes precedence over the CMakeCache-based value which is
dropped.

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