board/common: drop operator and staff, add backup user and group

- Free up 'operator' and 'staff' groups for NETCONF use
 - Reserve 'backup' user and group for future auto-backup (Debian)

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/rootfs/etc/group

@@ -16,10 +16,9 @@ video:x:28:
 audio:x:29:
 tape:x:32:
 www-data:x:33:
-operator:x:37:
+backup:x:34:
 utmp:x:43:
 plugdev:x:46:
-staff:x:50:
 lock:x:54:
 netdev:x:82:
 users:x:100:

board/common/rootfs/etc/passwd

@@ -5,5 +5,5 @@ sys:x:3:3:sys:/dev:/bin/false
 sync:x:4:100:sync:/bin:/bin/sync
 mail:x:8:8:mail:/var/spool/mail:/bin/false
 www-data:x:33:33:www-data:/var/www:/bin/false
-operator:x:37:37:Operator:/var:/bin/false
+backup:x:34:34:backup:/var/backups:/bin/false
 nobody:x:65534:65534:nobody:/home:/bin/false

board/common/rootfs/etc/shadow

@@ -5,5 +5,5 @@ sys:*:::::::
 sync:*:::::::
 mail:*:::::::
 www-data:*:::::::
-operator:*:::::::
+backup:*:::::::
 nobody:*:::::::