This follows how bitbake performs path insertion, and fixes a
failure to start wic on Ubuntu 15.10 with the distribution's
version of python-ply installed.
Signed-off-by: Matt Madison <matt@madison.systems> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>