Fancy Kernel r52

- fix /system mounting to ro with f2fs fstab
- add self-compiled stripped busybox with f2fs support
- re-add ROM-independent init.d support
- execute init.d through busybox's run-parts instead of sysinit (which is not available in every ROM, e.g. shiny; fixes root issues reported by some users)
- increase sched_rt_runtime_us marginally (~1% more realtime cpu usage granted for parent cpuctl group; likely not noticable?!)
- dont renice flush-daemon(s), they're already at +20 at default