TODO: merge make cold and the release script

[kit-ty-kate]

All three scripts (Makefile, release/Makefile and release/Dockerfile.in) compile the OCaml compiler, the version number isn't even kept in sync and it's all very annoying to deal with. On top of that they also all include a way to cold build and all do it in a slightly different way.

The proposal would be to:

• get rid of the duplicated scripts by creating a new make release target that would create a stripped (and static where possible) binary
• backport these change into the 2.1 and upcoming 2.2 branches so that the branch itself is used to build the releases instead of taking the release script from the latest commit from master. This would maybe imply that we want to move the part of the Makefile that deals with make cold-type rules on their own Makefile for easier sync between master and the maintenance branches