python-mako: use setuptools.bbclass, fixing install paths

An argument was missed in the manual setuptools configuration which meant that
the tool was installed into
/data/poky-master/tmp/sysroots/x86_64-linux/usr/bin/mako-render.  Using the
setuptools class reduces duplication and puts the tool into /usr/bin where it
belongs.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>