strip directory entries from package file list when offline installing .ipk archives,
this prevents opkg from removing directories like /usr later on when trying to
uninstall preinstalled packages (partial fix for #6772)


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19936 3c298f89-4303-0410-b956-a3cf2f4a3e73