init: Use a relative symlink for ueventd to init

make doesn't seem to handle the absolute symlink from
$OUT/root/sbin/ueventd to /init well - it follows the link during
dependency resolution, decides $OUT/root/sbin/ueventd doesn't exist
because /init doesn't exist, and relinks it every time.

Change-Id: I9ca1c14fe5fa80634f51ffc51a7c73146d29d42e