Strictly build what we need to deploy the unikernel and build block device into the default context

[dinosaure]

Currently, mirage build is equivalent to dune build --root .. In such case, we don't really know what we build. It will be nice to expand mirage build to what we really want to build such as: dune build --root . dist/unikernel.hvt for example.

The question becomes tricky for artifact such as block-device produced by something like tar or docteur: dune build --root dist/unikernel.hvt dist/disk.img. But, as far as I can say, we already collect into mirage_impl_block these block-device.

Then and a last point, it seems a non-sense to try to produce a block-device from the Solo5/cross-compilation context. Indeed, we probably want to produce some files like some *.js from some OCaml code and pack them into an image then. In that situation, js_of_ocaml does not play well with our cross-compilation toolchain and it seems more accurate to limit the production of block-devices into the default context (not the cross-compilation context).

If we run the production of block-devices into the default context AND we explicitly asks dune to produce our unikernel and block-devices, we finally are able to:

• prepend the production of our block-devices by some host computations
• still produce everything needed by our unikernel with a simple mirage build

Such design is not a big deal but I probably missed some implications and this is why I would like to discuss about that first. /cc @TheLortex who is the most aware about all of that.