intel_adsp: move `west sign` from `west flash` to earlier `west build`

Invoking `west sign` in `west build` accelerates twister because `west
build` is run in parallel, see rationale in superseded and very
different (CMake-based) PR #52942.

To maximize backwards compatibility:
- `west sign` is optional in `west build`
- `west flash` will sign (again) if any rimage --option is passed

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