main/busybox: upgrade to 1.31.0

Notable changes:

* udhcpc: upstream prefers -b over -n now. Since we set -b instead of -n
  by default this is not desired as it would not allow users to
  overwrite the -b without recompiling busybox. I therefore reverted the
  upstream commit introducing this change [0].

[0]: https://git.busybox.net/busybox/commit/?id=87e216294af9eec39c0c1d553555f8a98c15db38