Currently, the python3-package.mk framework relies on HOST_BUILD_PREFIX to
refer to the $(STAGING_DIR)/host directory but using the HOST_BUILD_PREFIX
variable requires the use of include/host-build.mk which in turn includes
python3-package.mk, leading to target redefinition errors.

In order to provide a global, uniform way to refer to the host staging
directory, LEDE introduced a new variable STAGING_DIR_HOSTPKG which points
to $(STAGING_DIR)/host for now with the purpose of eventually being able to
relocate that directory in the future.

This commit changes python3-package.mk to ...
 - stop including include/host-build.mk (revert of #3423)
 - replace usages of $(HOST_BUILD_PREFIX) with $(STAGING_DIR_HOSTPKG)
 - warn and fallback to $(STAGING_DIR)/host if STAGING_DIR_HOSTPKG is
   unavailable

The fallback code will ensure that the python host build infrastructure
continues to work properly on older OpenWrt and LEDE versions until the
STAGING_DIR_HOSTPKG is fully settled in and can be removed some time
in the future.

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