build: fix python load paths

Fixed python load paths so that vppcfg will work installed as python
library and standalone from the source directory, fixing load
pathes for resources such as yaml files along the way.

Added a make target for pylint called 'make check-style', fixed a
number of minor pylint issues along the way.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>

vppcfg/vppcfg.py

@@ -15,12 +15,20 @@
 # -*- coding: utf-8 -*-
 """vppcfg is a utility to configure a running VPP Dataplane using YAML
 config files. See http://github.com/pimvanpelt/vppcfg/README.md for details. """
+# pylint: disable=duplicate-code
+import os
 import sys
 import logging
 import yaml
-from config import Validator
-from vpp.reconciler import Reconciler
-from vpp.dumper import Dumper
+
+# Ensure the paths are correct when we execute from the source tree
+try:
+    from vppcfg.config import Validator
+except ModuleNotFoundError:
+    sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+    from vppcfg.config import Validator
+from vppcfg.vpp.reconciler import Reconciler
+from vppcfg.vpp.dumper import Dumper
 
 try:
     import argparse