kernel: improve initramfs bundle processing time

This is a partial fix for bugzilla 15059 [https://bugzilla.yoctoproject.org/show_bug.cgi?id=15059

]

It has been noted by several people that when an initramfs is bundled:

  - a lot of the kernel is rebuilt
  - it takes a really long time

When looking at the logs, the second kernel compilation (that performs
the bundle) is not using the parallel make settings, and builds with
-j1.

We are already explicitly passing PARALLEL_MAKE when building kernel
modules, and by extending that explicit use to the main kernel
compilation, we ensure that we always get a parallel build.

Build times chnaged from more than 30 minutes for the bundle, to
3 minutes in local testing.

The question of whether or not too much is rebuilding during the
bundle step is still an open question, but with this tweak, at least
the build time is back in the realm of acceptable.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
(cherry picked from commit 88fd394e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>