tell people which MirageOS version to use if they have the wrong one

[sabine]

[sabine]

Not 100% sure the detailed check for opam>=2.1.0 works in all compilation environments, please take a careful look, or be prepared to change to a simpler (but not fully accurate) opam>=2 check.

[hannesm]

Thanks for your motivating work. We have since in Mirage 4.4 introduced a header (* mirage >= 4.4.0 & < 4.6.0 *) for "config.ml" which outputs a nice error message if your mirage version is too old or too new. Obviously this only works since mirage 4.4.0.

In #395 I added these headers to all the example unikernels :)

[sabine]

Thank you, Hannes.