Split the px4_layer into user and kernel space libraries. Add some stubs for
user-space (e.g. version) where an interface to the kernel needs to be added
Use posix-versions for cpuload.cpp and print_load.cpp for userspace; these link to nuttx internals. This functinality could be built on top of posix (e.g. procfs) interfaces
Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>