opkg: indicate upgrade vs install

when running the postinst trigger we set PKG_UPGRADE=1 if this is an upgrade
and not an install.

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42468 3c298f89-4303-0410-b956-a3cf2f4a3e73