rules.mk: add STAGING_DIR_HOSTPKG variable

Add a STAGING_DIR_HOSTPKG variable which refers to $(STAGING_DIR)/host in order
to prepare support for relocating that directory in the future.

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

rules.mk

@@ -140,6 +140,7 @@ PKG_INFO_DIR := $(STAGING_DIR)/pkginfo
 
 BUILD_DIR_HOST:=$(if $(IS_PACKAGE_BUILD),$(BUILD_DIR)/host,$(BUILD_DIR_BASE)/host)
 STAGING_DIR_HOST:=$(TOPDIR)/staging_dir/host
+STAGING_DIR_HOSTPKG:=$(STAGING_DIR)/host
 
 TARGET_PATH:=$(subst $(space),:,$(filter-out .,$(filter-out ./,$(subst :,$(space),$(PATH)))))
 TARGET_INIT_PATH:=$(call qstrip,$(CONFIG_TARGET_INIT_PATH))