export MAKEOBJDIRPREFIX moved up in script

Dave,

First off, Thanks, for referencing me in the README.md file.  A friend at work started working on building an image for the Pi from source and said that your script was the best he had found to do the build and create the image.  He also said that I was mentioned.  I had forgotten about that post. Again, thanks.  Brian and I have been working with FreeBSD since back in the version 3 days.

This patch moves MAKEOBJDIRPREFIX above the setting of IMG.  I'm not sure it that was an oversight or I'm just reading the code wrong.  Without the patch IMG will end up being /bsd-pi.img as opposed to /src/FreeBDSD/obj/bsd-pi.img.

Thanks for considering this patch.