NuttX exported archive hard to use with CMake

[leducp]

The exported archive is hard to use with an external CMake application: one who wants to use it must write its own toolchain file dedicated to its project. Such toolchain file must be adapted every time NuttX build change (like switching to another target or version upgrade).

On the other hand, the exported archive contains a Make.defs that contains everything needed to generate a generic toolchain file so it should be possible to add a generic CMake toolchain file in the archive.