Some people might want to run the gitlab CI pipelines in an environment
where multiple CPUs are available to the runners, so let's rather get
the number for "-j" from the "nproc" program (increased by 1 to compensate
for jobs that wait for I/O) instead of hard-coding it.

Message-Id: <20200525131823.715-7-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>