Allow style checking of tree and local changes

Allow style checking of tree and local changes

New phony Makefile targets have been added:

 * checkcodebase
 * checkpatch

The checkcodebase target will run a Linux style compliance check over the
entire codebase, and honours the V=1 Makefile verbose setting and so will
show more information when this is enabled.

If the local directory is a git checkout then the output of git ls-files is
used to decide which files to test for compliance.  If the local directory
is not under git control then a 'best attempt' is made, but in this case it
should be noted that it is possible for additional non-codebase files to be
tested, so care should be taken when parsing the output.

The checkpatch target will compare local changes against the git origin/master
to allow issues with the last set of changes to be identified.  To override
the change comparision location, set the BASE_COMMIT variable to your
desired git branch.

Both targets rely on the Linux source tree script checkpatch.pl to do the
syntax checking, and expects that the CHECKPATCH environment variable points
to the location of this file.

Notes on the usage of these targets have been added to the contributing.md
and docs/user-guide.md text files.

Change-Id: I6d73c97af578e24a34226d972afadab9d30f1d8d