Support for hardcoding argv in the manifest

[mkow]

Description of the problem

We currently have loader.argv_src_file for reading hardcoded args from a file, but would be nice to have a way to specify them directly in the manifest in case they are short and simple. I originally couldn't add this when implementing loader.argv_src_file, because back then we didn't use TOML and the old syntax was too primitive to be able to support such features.

Proposed syntax

loader.argv = ["arg0", "arg1", "arg2"]

Specifying more than one from {loader.argv, loader.argv_src_file, loader.insecure__use_cmdline_argv} should be treated as an error.
Not sure whether loader.argv0_override will still be useful after this change, I don't really remember what was the intended use case (something with busybox probably?).

[oshogbo]

I can look into this :)