10x speedup of disk image building

By using a swap based md this seems to speed up our image generation by an order of magnitude.

It shaves about 10 minutes off our build. 

Warning: you want a bunch of swap on your build machine otherwise things will explode.  Might be a good option for "git-internal" builds only?

Prior:
00:04:38 ### log: /usr/home/alfred/freenas/os-base/amd64/_.di
00:17:01 ## Compressing GUI upgrade image

With this patch:
00:05:30 ### log: /usr/home/alfred/freenas/os-base/amd64/_.di
00:07:17 ## Compressing GUI upgrade image