Python: support simultaneous Python2 and Python3

[MilhouseVH]

Python 2.7.14 changelog
Python 3.6.3 changelog

Buildroot Python3 patches: https://github.com/buildroot/buildroot/tree/master/package/python3

Python3:host is going to be needed to build systemd-235 with meson.

Python3:target is not yet required, but longer term Kodi may switch to Python3, so this also gets us some of the way there.

Samba will continue to use only Python2 for the foreseeable future.

In general this PR:

• Removes hard-coding of python2.7 - this is now determined from the relevant Python{2,3} package
• Default $TOOLCHAIN/bin/python will be switched automatically according to Python{2,3} dependency

[lrusak]

I have to add this patch to dbus-python to get it to build. I'm not sure why but it is the only package that had a problem

diff --git a/packages/python/system/dbus-python/package.mk b/packages/python/system/dbus-python/package.mk
index 02c1c0a..afb3525 100644
--- a/packages/python/system/dbus-python/package.mk
+++ b/packages/python/system/dbus-python/package.mk
@@ -32,6 +32,7 @@ PKG_IS_ADDON="no"
 PKG_AUTORECONF="no"
 
 pre_configure_target() {
+  export PYTHON_CONFIG="$SYSROOT_PREFIX/usr/bin/python2-config"
   export PYTHON_INCLUDES="$($SYSROOT_PREFIX/usr/bin/python2-config --includes)"
   export PYTHON_LIBS="$($SYSROOT_PREFIX/usr/bin/python2-config --ldflags)"
 }

[MilhouseVH]

Updated.

[lrusak]

Great work!