west: rimage: generate TOML with all LLEXTs included

"west sign" uses the C preprocessor to generate a TOML configuration
file, needed for rimage. When creating LLEXTs rimage also uses a
configuration file where all supporting components are configured as
LLEXT. This adds such a file generation.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>