fs/squashfs: honour the number of parallel jobs

fs/squashfs: honour the number of parallel jobs

By default, mksquashfs uses all the CPUs available, like us with
BR2_JLEVEL=0.

However, by setting BR21_JLEVEL to non-zero, a user volutarily limits
the number of jobbs allowed to run concurrently (for various reasons,
like this is not a priority task on the machine; or like the
autobuilders, to spread the load accross multiple runs without
overloading the machine).

So, propagate tthe parallelisn settings down to mksquashfs, to repect
the user's will.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Tested-by: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>