Allow specification of license in settings file for linux deb files

[josh-richardson]

Currently, output from dpkg --info AppName.deb specifies that the license of the package to be installed is unknown. This small change will allow users to specify the license in the deb file, based on a variable in a config file.

A user of fbs would add the line "license": "GPL" or similar in their linux.json to use this feature.

[mherrmann]

Thanks! I haven't tested it, but from reading the code could it be that fbs installer fails on Linux if you don't have license set in a settings file such as linux.json? If yes, then I believe we should add "license": "" to _defaults/.../linux.json.

[josh-richardson]

Thanks! I haven't tested it, but from reading the code could it be that fbs installer fails on Linux if you don't have license set in a settings file such as linux.json? If yes, then I believe we should add "license": "" to _defaults/.../linux.json.

We are checking here that license exists in the JSON, but there is the situation whereby license exists in the file, but has not yet been specified, this may cause a failure I suppose. I'll update this PR to be a little more robust.