prefix init will not create nested directories

[dkozel]

In prefix init sysutils.mkdir_writable is used https://github.com/gnuradio/pybombs/blob/master/pybombs/commands/prefix.py#L266

rather than mkdirp_writable. https://github.com/gnuradio/pybombs/blob/master/pybombs/utils/sysutils.py#L77

The result is that pybombs prefix init ~/prefix/default/ will fail if ~/prefix does not already exist.

Which behavior is the better one?