- Set BUSYBOXDIR empty initially and let busybox's conf hook override it
- Fail if BUSYBOX=y and BUSYBOXDIR remains empty after running conf hooks
- Bump the minimum version to the first version that has the conf hook
- Update the automatic selection of klibc utilities to check whether
  BUSYBOXDIR is empty

Closes: #810154
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>