The main changes introduced are as follows:

- Xenomai/cobalt core (nucleus) moved at top level, with the arch-dep
bits, POSIX and RTDM personalities in sub-directories.

- Renamed include/cobalt/nucleus to include/cobalt/kernel.

- include/asm-* directories moved to include/cobalt/.

- Removal of asm-<arch>/bits, with most code reintroduced as regular
  implementation files into lib/cobalt/sysdeps/<arch>/.