Bitbake does really weird things with negative or zero numbers of threads
which is confusing to the user. Add a sanity check for this.
When you have code doing arithmetic on the values and a VM reconfigures
to only a single thread, negative numbers are easier than you'd think.
(Bitbake rev: 32166ac3c85ff3c04081580ae76bd63590d6ff3e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>