west: sign.py: give rimage a key placeholder when there's none

rimage always requires a key argument even when it does not use it.

Some configurations (IMX) use rimage only for "stitching" the image but
not for signing. These were failing to build directly from west with
pretty cryptic stack trace, see below. This has never been a problem for
the SOF project because it has always worked around this rimage
limitation by providing a bogus -k argument - very much like the one
added by this commit.

```
File "zephyrproject/zephyr/scripts/west_commands/sign.py",  in sign
  extra_ri_args += [ '-k', str(sof_src_dir / 'keys' / cmake_default_key) ]
TypeError: unsupported operand type(s) for /: 'PosixPath' and 'NoneType'
ninja: build stopped: subcommand failed.
```

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

scripts/west_commands/sign.py

@@ -522,7 +522,8 @@ class RimageSigner(Signer):
         sign_config_extra_args = config_get_words(command.config, 'rimage.extra-args', [])
 
         if '-k' not in sign_config_extra_args + args.tool_args:
-            cmake_default_key = cache.get('RIMAGE_SIGN_KEY')
+            # rimage requires a key argument even when it does not sign
+            cmake_default_key = cache.get('RIMAGE_SIGN_KEY', 'key placeholder from sign.py')
             extra_ri_args += [ '-k', str(sof_src_dir / 'keys' / cmake_default_key) ]
 
         if '-c' not in sign_config_extra_args + args.tool_args: