Cosmetic change: Omit .config file extension from -config argument

[weiss]

Specify -config sys rather than -config sys.config. This is what the erl(1) manual says, it's what other release generators do, and it saves a few characters on the process command line :smile:

However, as both sys and sys.config will work, this fixes a non-issue, so feel free to just close the PR if you don't like the change.

[tsloughter]

Probably not going to accept this because, ironically, it actually adds .config all over the files :). I think it will just be confusing to see that when reading the scripts, esp if it isn't immediately recognizable as removing instead of adding the prefix.