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_2.7.15.bb

@@ -21,7 +21,7 @@ SRC_URI += "\
   file://setuptweaks.patch \
   file://check-if-target-is-64b-not-host.patch \
   file://search_db_h_in_inc_dirs_and_avoid_warning.patch \
-  file://avoid_warning_about_tkinter.patch \
+  ${@bb.utils.contains('PACKAGECONFIG', 'tk', '', 'file://avoid_warning_about_tkinter.patch', d)} \
   file://avoid_warning_for_sunos_specific_module.patch \
   file://python-2.7.3-remove-bsdb-rpath.patch \
   file://run-ptest \
@@ -45,6 +45,7 @@ EXTRA_OECONF += "ac_cv_file__dev_ptmx=yes ac_cv_file__dev_ptc=no"
 
 PACKAGECONFIG ??= "bdb"
 PACKAGECONFIG[bdb] = ",,db"
+PACKAGECONFIG[tk] = ",,tk"
 
 do_configure_append() {
 	rm -f ${S}/Makefile.orig
@@ -172,7 +173,7 @@ RDEPENDS_${PN}-modules += "${PN}-misc"
 
 # ptest
 RDEPENDS_${PN}-ptest = "${PN}-modules ${PN}-tests unzip"
-
+RDEPENDS_${PN}-tkinter += "${@bb.utils.contains('PACKAGECONFIG', 'tk', 'tk', '', d)}"
 # catch manpage
 PACKAGES += "${PN}-man"
 FILES_${PN}-man = "${datadir}/man"