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

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>