opkg-wrapper: allow ignoring custom feeds

This adds ignore list for opkg-wrapper. It is implemented using Lua pattern matching function. If one of provided patters is matching given line in customfeeds.conf then feed from such line is ignored. Source for used patterns are files in a directory '/etc/updater/opkg-ignore'. Every line is a patterns with exception of those starting with '#', those are intended for commenting.

There is a situation where custom feed is added and to allow it to be easily used it is added not only to opkg but also to updater. In such case such feed is pulled twice because of opkg-wrapper. This would not be huge problem unless there are some additional settings and both downloads are not in fact same. This can decrease security by going around of more secure updater's configuration and less secure URL from opkg-wrapper.

Unfortunately we can't do this automatically because there is no access to already known repositories. And even if such access would existed we couldn't be sure that repository we are adding is not going to be added later then opkg-wrapper is executed. Because of that much more better solution is to have some sort of ignore list for opkg-wrapper.