Remove branch decision logic from build.sh and hardcode it in feeds.c…

Remove branch decision logic from build.sh and hardcode it in feeds.conf.mfw

This brought unnecessary pain in multi-branch official builds. Now
feeds.conf.mfw will differ for instance between master openwrt-18.06
branches, and it will be the only source of truth when answering the
question "what branch is each feed built from ?".

If you want to build feeds from other branches, you can edit
feeds.conf.mfw. If you're afraid to forget about that local edition,
which could later result in a commit and push of that file, you can
instead use an out-of-tree buildtools; the following sequence
will effectively use /tmp/mfw-mine/feeds.conf.mfw:

  cp -r mfw /tmp/mfw-mine
  emacs -nw /tmp/mfw-mine/feeds.conf.mfw
  /tmp/mfw-mine/build.sh -d wrt3200 -l musl -v master