For pxz, only use "-T 16" if the number of CPU's is greater than or equal to 16.
Use hw.ncpu for lower numbers of CPU's.  Without this change,
on some desktops and laptops, compiling FreeNAS will fail due
to the large amount of memory per thread used by pxz with higher "-T" values.

This was discovered by Patrick Qi trying to compile FreeNAS on a laptop
with 16GB of RAM and hw.ncpu=4, and I reproduced the problem compiling on a desktop
with 16GB of RAM and hw.ncpu=4.
The .usb image must be copied to the release location when doing "make release".

Ticket: #5184