[FEATURE] rimage should be able to re-sign FW with different key

[lgirdwood]

Is your feature request related to a problem? Please describe.
Currently we need to rebuild FW ELF file to sign with a different key (impossible if FW comes as a binary and is missing some sources or compiler tools). It should be possible for rimge to re-sign so that users can take community/intel/vendor signed releases and resign them for debug or for running on other hardware.

Describe the solution you'd like
A new cmd line switch to rimage that only invokes the signing flow for an existing signed binary.