Add a way to add a command to the build/install steps of a binary package

[lefessan]

Currently, binary packages only install files. Sometimes, it would be useful to be able to execute a command, for example for conf-XXX packages that could still want to check if the external dep is present and fail otherwise.

It would be interesting to find a way to do that.
It should be done in three places:

• define a way for the relocation patch or the package to define these commands (opam-bin-run.sh maybe ?)
• add it to the generated opam file for the binary package and save it somewhere
• also call it when using a cached binary package instead of a source package