python: add tk support

Add support to enable tk via PACKGECONFIG.
before the patch:
 # python
 Python 2.7.15 (default, Nov  8 2018, 04:53:50)
 [GCC 8.2.0] on linux2
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import Tkinter
 Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/lib-tk/Tkinter.py", line 39, in <module>
    import _tkinter # If this fails your Python may not be configured for Tk
 ImportError: No module named _tkinter
 >>>

After the patch, if enable tk in PACKGECONFIG, then
 # python
 Python 2.7.15 (default, Oct 25 2018, 08:12:45)
 [GCC 8.2.0] on linux2
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import Tkinter
 >>>

(From OE-Core rev: 26b2702475697c123937d237e00c34eb2216b5ea)

Signed-off-by: Mingli Yu <Mingli.Yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python/python2-manifest.json

@@ -966,6 +966,7 @@
             "core"
         ], 
         "files": [
+            "${libdir}/python${PYTHON_MAJMIN}/lib-dynload/_tkinter.so",
             "${libdir}/python2.7/lib-tk"
         ]
     }, 
@@ -1043,4 +1044,4 @@
             "${libdir}/python2.7/lib-dynload/zlib.so"
         ]
     }
-}
+}