package: replace $(STAGING_DIR)/host with $(STAGING_DIR_HOSTPKG)

Cleanup to prepare for changing STAGING_DIR_HOSTPKG. The actual change of
STAGING_DIR_HOSTPKG (i.e., moving the host packages back into a common, not
target-specific directory) will be done after the first LEDE release, but
the cleanup will also be useful for projects like Gluon.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>