Solo5 core platform libraries for MirageOS

This package provides the MirageOS OS library for Solo5 targets, which handles the main loop and timers. It also provides the low level C startup code and C stubs required by the OCaml code.

Currently this package also includes the C stubs used by the Solo5 console, block and net implementations.

The OCaml runtime and C runtime required to support it are provided separately by the ocaml-freestanding package.