Add ability to freeze list of iotile plugins

When running on an SBC with an SD card, enumerating the filesystem inside `pkg_resources` to find iotile plugins is very slow, adding 5-10 seconds to the start up time of any `iotile` command line script. It's not possible to actually speed up `pkg_resources` but in situations where we know that the user is not installing / removing plugins (i.e. when the file system is read only), we can prescan all installed plugins and produce an `entry_points.txt` file similar to the component registry that contains all of the entry points.

If this file exists it can be consulted before `pkg_resources` is imported to speed up application load time:

See `pkg_resources` slow behavior, known bug, here:
https://github.com/pypa/setuptools/issues/926