python: add include guard for python-host.mk and include in Makefile

So that it can be included twice.
python-package.mk include python-host.mk, but at some point
in time python-host.mk is not exported to staging_dir yet.

So, to have in python's Makefile, we need to include it
in the Makefile, and to prevent double inclusion (if
python-host.mk is exported from a previous build), the
include guard is added.

Also, moved the includes after some vars were set.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/Makefile

@@ -7,6 +7,9 @@
 
 include $(TOPDIR)/rules.mk
 
+# This file provides the necsessary host build variables
+include ./files/python-host.mk
+
 # The file included below defines PYTHON_VERSION
 include ./files/python-package.mk