base-file: remove password aging feature form /etc/shadow

In the default shadow file, as visible in the failsafe mode, the user
root has value of `0` set in  the 3rd field, the date of last password
change. This setting means that the password needs to be changed the
next time the user will log in the system. `dropbear` server is ignoring
this setting but `openssh-server` tries to enforce it and fails in the
failsafe mode because the rootfs is R/O.

Disable the password aging feature for user root by setting the 3rd
filed empty.

Signed-off-by: Rucke Teg <rucketeg@protonmail.com>

package/base-files/files/etc/shadow

-root::0:0:99999:7:::
+root:::0:99999:7:::
 daemon:*:0:0:99999:7:::
 ftp:*:0:0:99999:7:::
 network:*:0:0:99999:7:::