package/canfestival: don't build in parallel

The Makefiles for canfestival are not correctly written, which leads to
multiple warnings such as:

    make[4]: warning: jobserver unavailable: using -j1. Add `+' to parent make rule.

Since canfestival is relatively small, it builds in less than 6s here
when not in parallell, while a parallel build takes 5s.

Just disable parallel build to avoid future surprises.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>