boost: Limit PARALLEL_MAKE to -j 64

:Release Notes:
bjam doesn't support more then 64 parallel jobs

:Detailed Notes:
bjam was segfaulting when people tried to build boost with -j greater
then 64, this .bbappend does the same as:
http://lists.openembedded.org/pipermail/openembedded-core/2013-June/080344.html
When PARALLEL_MAKE is set to greater number it will use just 64 threads.
It cannot be added to meta-webos-backports, because it wasn't merged in
oe-core yet.

:Testing Performed:
boots builds OK with -j 99 and uses 64 threads
bitbake -e confirms that BOOST_PARALLEL_MAKE was set to -j 64
bitbake -e confirms that PARALLEL_MAKE = "-j 99 something" works
correctly too

:QA Notes:

:Issues Addressed:
[GF-6959] boost-1.53 build segfaults when building with -j greater than 64

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>

Change-Id: I85a2a24586790f6f4fd61f2123dca737f6c4288b
Reviewed-on: https://g2g.palm.com/2770
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>