Move the python-host.mk and python-package.mk includes after the definition of
common package variables. This is required to ensure that PKG_UNPACK is not set
to to the dummy "true" command which happens if PKG_SOURCE is not yet set.

Fixes the following error observed while attempting to build Python on a recent
LEDE version:

    make[2]: Entering directory '.../lang/python'
    true
    [ ! -d ./src/ ] || cp -fpR ./src/* .../Python-2.7.12

    Applying ./patches/001-enable-zlib.patch using plaintext:
    can't find file to patch at input line 14
    Perhaps you used the wrong -p or --strip option?
    [...]
    Patch failed!  Please fix ./patches/001-enable-zlib.patch!
    Makefile:242: recipe for target '.../Python-2.7.12/.prepared_...' failed

Signed-off-by: Jo-Philipp Wich <jo@mein.io>