[busybox] use SHA-512 ($6$...) passsword encryption by default, like
the passwd program from the "shadow" package already does.

This allows for replacement of the "shadow" package by busybox
components at some time. It also prevents users from creating
uids with DES encryption which supports passwords only < 8 characters.

Signed-off-by: Andy Blackburn <andrew.blackburn644@btinternet.com>