Increase dom0 memory to 256M

Without this `apt-get install openssh-server` is excruciatingly slow.
Point to xen-arm-builder for boot.cmd

This takes care of the synchronization of the blog post with the
xen-arm-builder.  We also point the the correct file for the
Cubietruck.
Merge pull request #213 from Chris00/mem

Increase dom0 memory to 256M