Move copiesto dockerfile

I wanted to create a pxe image as well as an iso where the kernel boot params are configurable.  Unfortunately, both of these things require copying the make_iso.sh, making my changes in that, effectively duplicating the boot2docker build.  Moving the cp lines from make_iso.sh to boot2docker/Dockerfile lets you layer any customization on top using the FROM directive in your own custom Dockerfile without any duplication.  You only need to add the From line and the RUN /make_iso.sh line to make a custom iso.
Why `COPY` this again, and not just do `RUN cp $ROOTFS/etc/version /tmp/iso/version` ?