west: sign: add new option -D for configurations

rimage signing tool requires the path to the configuration directory.
This can be fetched using cmake cache if we are building with a module
that has rimage integrated like the sof module, however, we should be
able to sign images that were built standalone and without SOF, so
support this new option to provide the location of the configuration
files for the tool.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>