Makefile: Force make to run serially in the top-level Makefile

The top-level Makefile can be executed in parallel, as it causes problems.
We can force make to be not parallel.

It's been reported many times, and recent discussions on IRC with kos_tom,
and user nick-named knee, led to this patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>