package/ipmiutil: disable parallel build

ipmiutil has a race when generating the object files and the executable,
because it uses very odd (to say the least) constructs in its
Makefile.am, leading to improper dependency-tracking.

Since impmiutil is pretty small, and because it is just mindbloggingly
complex to fix its Makefile.am, just disable parallel build.

Fixes:
    http://autobuild.buildroot.org/results/661/661373306d0440aaea67e394fbbfc47fd4481589/
    http://autobuild.buildroot.org/results/a6c/a6c5f9d8e0e0bad27b8ca1fa217bfda4216f2cd8/
    http://autobuild.buildroot.org/results/f0b/f0bbbd54349b5239872d6de185e16c1d90b3f779/
    http://autobuild.buildroot.org/results/20c/20c09695481c3d1030852330c1d1f1f9489f1f41/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>