scripts: Build in parallel if requested

Instead of passing -jn directly through to make, and then building
each target in sequence, use the -j and -l options to determine a
number of threads to perform parallel invocations of make in.

The build results are collected, and printed when each build finishes,
which eliminates the interleaving that would otherwise occur between
builds.

Change-Id: Ieef0dcc8adfc72cbe043a3491d4d6f6c23fede31
Signed-off-by: David Brown <davidb@codeaurora.org>