pxe: Allowing pigz to compress initrd

When developping edeploy, creating initrd is pretty long. This patch is
a first try to use pigz when available.

On a Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz with 8G of memory :
gzip : 1m21
pigz :   21 seconds

The speed is up to 4 here while I'm running 4 logical cpu units.
Sounds very promising ;o)