waf: Remove as many sys path hacks as we can.

There are still sys.path hacks in the conf.py files. They cannot
be removed because the instance of python running the sphinx-build
command does not see our top level path hacks. I looked at using
PYTHONPATH but I could not figure out how to set a process environ
var for a waf build instance.

wscript

@@ -5,17 +5,23 @@
 import sys
 import os.path
 
-sys.path.insert(0, os.path.abspath('common'))
-
 import waflib
-import waf as docs_waf
-import version
+
+from common import waf as docs_waf
+from common import version
 
 #
 # Branch version
 #
 rtems_major_version = '5'
 
+#
+# Set Python's system path to `common` from the top level so the
+# conf.py modules in subdirectories can be found. See xml_catalogue in
+# common/waf.py.
+#
+sys.path.append(os.path.abspath('common'))
+
 #
 # The documents to build.
 #