genboardscfg: limit to 240 jobs

When genboardscfg.py is run on machines with 255 or more cores, the
process will consume more than 1024 file descriptors, which is a common
standard ulimit for user processes. As a consequence it will fail with a
lenghty Python trace, with the almost hidden message:
OSError: [Errno 24] Too many open files

It's somewhat questionable whether that level of parallelity is actually
useful for genboardscfg, so we limit the *default* number of jobs to the
safe number of 240, to avoid the problem.
If a user persists, she can still force a higher number via the -j
parameter - hopefully having raised the ulimit accordingly beforehand.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>