rules.mk: export STAGING_DIR_HOST to sub-makefiles and shells

Certains host packages (like quilt) need to know the previously used
install location at run-time. They currently use $STAGING_DIR, which
causes issues on cleaned environment, where $STAGING_DIR doesn't exist
yet.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47409 3c298f89-4303-0410-b956-a3cf2f4a3e73