/etc/writable should be handled by the initrd, not by rootfs builds

[Oliver Grawert]

currently we set up linking in /etc/writable during image build for files that need atomic writes (on the phone thats only three, on snappy it seems to increase a lot).
doing it from livecd-rootfs is hackish and will bind us forever to that image builder (or at least require to move such hacks forward and maintain them in a potentially new build system). instead this should be set up from teh initrd during boot, after the writable mounts are in place, this is a one time thing (should check for existing linkage and just bail out then)