Merge pull request #158 from avsm/master

Fix HTTP/cohttp dependencies in autoinstall
Merge pull request #159 from avsm/master

Always call ocamlbuild on every `make` by declaring the main target PHON...
Merge pull request #160 from avsm/master

Support `Fat_KV_RO` (a read-only k/v version of the FAT filesystem).
Fix the Mirage HTTP compilation problem by using the new split-out
libraries for `mirage-http-unix` and `mirage-http-xen`.

Closes mirage/mirage#161