treewide: replace $(STAGING_DIR)/host and $(HOST_BUILD_PREFIX) with $(STAGING_DIR_HOS...

treewide: replace $(STAGING_DIR)/host and $(HOST_BUILD_PREFIX) with $(STAGING_DIR_HOSTPKG)

As both LEDE and OpenWrt have STAGING_DIR_HOSTPKG now, we can start to rely
on it. See 73b7f55424de52d8179a9ad808252fe3bf8dcc9d for more information on
STAGING_DIR_HOSTPKG.

STAGING_DIR_HOSTPKG won't actually be changed before the first LEDE release
(it is equivalent to $(STAGING_DIR)/host), so this simple search/replace
cleanup is safe to apply. Doing this cleanup now will be useful for the
Gluon project (an OpenWrt/LEDE based firmware framework) for experimenting
with modifying STAGING_DIR_HOSTPKG before doing this in the LEDE upstream.

Also fixes a typo in the dbus Makefile ("STAGIND_DIR").

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