rtemstoolkit: Add unit testing for the python modules

- Add support to run the unit tests for the rtemstoolkit python
  modules from waf. Enter './waf test' for the tests to be run on
  python2 and python3.
- Update the importing of rtemstoolkit modules to the standard
  method which works on python2 and python3.
- Update the README.

rtemstoolkit/freebsd.py

@@ -35,16 +35,8 @@
 
 import os
 
-#
-# Support to handle use in a package and as a unit test.
-# If there is a better way to let us know.
-#
-try:
-    from . import check
-    from . import execute
-except (ValueError, SystemError):
-    import check
-    import execute
+from rtemstoolkit import check
+from rtemstoolkit import execute
 
 def cpus():
     sysctl = '/sbin/sysctl '