Add support to install files/symlinks in parallel.

Add support to install files/symlinks in parallel.

This reduces the install-zst target time from 100 s to 25 s on my test
system, a 400% speed improvement.

* Makefile (NUM_JOBS): New variable.
(install, install-xz, install-zst): Use it.
* copy-firmware.sh (num_jobs): New variable.
(has_gnu_parallel): New procedure.
<-j>: Parse new option, and use it along GNU parallel to parallelize
firmware copying and compression/symlink creation.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>