Extend the "add url" function to allow file:// URLs

This allows URLs of the form file:///path/to/local/file.conf to be used
in nc-add-url, in order to access configuration files relative
to the root directory. This is primarily a debugging tool aimed at
developers rather than an expected use case.

The DEVICE_TYPE_ANY enum is used in this case to represent that a
resulting boot option is not associated with any device in the
traditional sense, and in the UI is represented as a "Custom Local
Option".

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>