- add sysupgrade support for keepfile hints
* introduces /lib/upgrade/keep.d/ for per-package keepfile lists
* introduces /etc/sysupgrade.conf for user defined keepfile hints
- prime /lib/upgrade/keep.d/base-files-essential to keep sysupgrade usable for images without opkg
- change sysupgrade to build the keepfile list from /lib/upgrade/keep.d/, /etc/sysupgrade.conf and
opkg list-changed-conffiles
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23258 3c298f89-4303-0410-b956-a3cf2f4a3e73
