ncp: Network Control Processor (NCP) implementation (#31) (#45)

In addition to the "SoC/CLI" implementation that is already included,
OpenThread needs a Network Control Processor (NCP) implementation to
facilitate applications where the Host Controller is logically
separate from the controller that implements the Thread stack (Which
is the NCP). This task also necessitates the definition of the
protocol that the Host Controller and the NCP use to communicate with
each other.

This commit represents a functional starting point for both of these.
The initial name of the line protocol is "Spinel" (Named after the
[gemstone][1]). The long-term goal is to standardize the Spinel
protocol to be the official line protocol for host-to-NCP comunication
for Thread.

A [draft of the Spinel protocol document][2] is included in this
commit. At this point everything is subject to change, but as-is the
protocol has been demonstrated to work fairly well.

The NCP implementation in the has been tested and is working well
enough for basic connectivity, but it is far from complete. A Host
implementation is not yet provided in the tree, but will eventually be
included. A separate host implementation for Linux has been written
and is anticipated to be released as a separate project over the next
few weeks.

[1]: https://en.wikipedia.org/wiki/Spinel
[2]: https://github.com/openthread/openthread/blob/feature/ncp-spinel/src/ncp/PROTOCOL.md#spinel-host-controller-interface

examples/Makefile.am

@@ -33,6 +33,7 @@ include $(abs_top_nlbuild_autotools_dir)/automake/pre.am
 DIST_SUBDIRS                            = \
     platform                              \
     cli                                   \
+    ncp                                   \
     $(NULL)
 
 # Always build (e.g. for 'make all') these subdirectories.
@@ -40,6 +41,7 @@ DIST_SUBDIRS                            = \
 SUBDIRS                                 = \
     platform                              \
     cli                                   \
+    ncp                                   \
     $(NULL)
 
 # Always pretty (e.g. for 'make pretty') these subdirectories.