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 73b7f55424 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>

net/prosody/Makefile

@@ -50,7 +50,7 @@ define Build/Configure
 	# this is *NOT* GNU autoconf stuff
 	(cd $(PKG_BUILD_DIR); ./configure \
 		--prefix=/usr \
-		--with-lua="$(STAGING_DIR)/host/bin" \
+		--with-lua="$(STAGING_DIR_HOSTPKG)/bin" \
 		--with-lua-include="$(STAGING_DIR)/usr/include" \
 		--with-lua-lib="$(STAGING_DIR)/usr/lib" \
 		--cflags="$(TARGET_CFLAGS)" \