Hack to allow building under OPAM

OPAM doesn't preserve symlinks, so readlink doesn't work.
Fixes for networking

This allows mirage-skeleton/network to handle TCP calls on x86_64 with
the new Mini-OS.
Create a thread for OCaml

Otherwise, it uses the tiny bootstrap stack (and schedule probably
doesn't work).
Hack: Replaced Xen checksum code with Unix version

The Xen version does unaligned double-word accesses, which fault on ARM.