stub: Detect empty LoadOptions when run from EFI shell

stub: Detect empty LoadOptions when run from EFI shell

The EFI shell will pass the entire command line to the application it
starts, which includes the file path of the stub binary. This prevents
us from using the built-in cmdline if the command line is otherwise
empty.

Fortunately, the EFI shell registers a protocol on any images it starts
this way. The protocol even lets us access the args individually, making
it easy to strip the stub path off.

Fixes: #25201