openblas: disable OpenMP

openblas: disable OpenMP

For powerpc (and powerpc only), the openblas build system will enable
USE_OPENMP when threads are enabled. But the toolchain wasn't
necessarily built with OpenMP support. So explicitly disable OpenMP -
for all architectures, not just powerpc, just to keep things simple.

We could have autodetected whether OpenMP is available at configure
time, but that would be more of a feature patch, it's up to upstream
to do that.

Fixes:
http://autobuild.buildroot.net/results/423b5e79cd4342d6c160ed478054b294b0826c6a
http://autobuild.buildroot.net/results/c9e554c8f880b49b3c9203725ac5e6565b7e5c6f

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>