Busybox in default uses SHA512 as well.

On big ditribution this default is sourced from PAM. That means that
shadow reads pam settings and uses that. OpenWrt in most cases does not
have PAM installed and in such case shadow fallbacks to its own default
which is DES. This just changes that default to SHA512 which is
consistent with rest of the system.

Signed-off-by: Karel Kočí <karel.koci@nic.cz>