canfestival: fix build failure

Some canfestival python build scripts are not python2 compliant.
So, this patch add a patch allowing to set the python interperter and
set it in the make environment. This pulls a host-python dependency on
canfestival.

This patch has been sent upstream: [1].

Fixes:
  http://autobuild.buildroot.net/results/98d/98dce833dfa8255522af672d4f41c2504c00e64e/

[1] http://sourceforge.net/p/canfestival/mailman/message/32919038/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>