nut: disable parallel builds

The build system has a race condition causing build failure,
so ensure make -j1 is used.

Fixes:
http://autobuild.buildroot.net/results/0dd/0ddd856bcbec2db0500791fd428ba053d6e4fa1b/build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>