The former nucleus directory becomes the root of all dual-kernel
specific kernel code as "cobalt", which also contains the posix,
native and rtdm sub-directories.

Rationale: the base kernel code we should provide is now well-bounded,
split as follows:

- the dual kernel bits which include the nucleus and the few remaining
  core interfaces, we will use to iron a non-rt vanilla kernel for
  delivering real-time capabilities.

- the native linux based code, basically the RTDM interface ported to
  the vanilla kernel.

At this chance, we may group all dual kernel bits inside the same tree
hierarchy since we now have a well-defined and bounded base kernel
support which should not significantly grow over time (this reasoning
obviously excludes RTDM-based drivers). As a matter of fact, only the
existing core interfaces should live in kernel space, all others
should live in userland over the copperplate library, so that we can
use them over vanilla (rt or even non-rt) kernels as well.

The same way, the RTDM implementation over vanilla kernel services
should be rooted under a future "mercury" directory at some point.