Create a bootable usb install image.

You can now type "make bootusb" at the top level and get a bootable image for installing FreeNAS.

This can be useful reinstalling a toasted FreeNAS box or installing on machines without CDROM or ipmi cdrom.

**Edit**
FYI: I know this patch isn't very clean, I would like feedback on how it was done before making it extremely tidy in case there was something I missed.  In particular the formatting is all jacked in create_iso.sh and create_bootusb.sh now, however I'm more than glad to clean that up if the main code path looks OK.