shadow: change to use SHA512 password encryption

shadow: change to use SHA512 password encryption

The default encryption method for shadow is DES, which limits passwords
to 8 characters. Not only is this undesirable, it's also not how busybox
works so we had different passwd/login length behaviour depending on
whether shadow was installed in the image or not. Change it to SHA512
which is what most Linux distributions seem to be using currently.
(SHA512 also matches up with how we are configuring PAM.)

Fixes [YOCTO #5656].

(From OE-Core rev: a9e072f9f0da774411e07abf47dd4bd8c6d685d7)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>