group file: define groups expected by udev

udev-182 requires several groups to be resolvable at udev startup,
including disk, cdrom, floppy, tape, audio, video, lp, tty, dialout,
and kmem.  Only some of these are in the default skeleton's group
file, So let's add the missing groups, and plugdev too.

This avoids getting these logs in /var/logs/messages:

Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'dialout' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'kmem' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'video' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'lp' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'floppy' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'cdrom' unknown
Jan  1 00:00:08 buildroot daemon.err udevd[37]: specified group 'tape' unknown

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

system/skeleton/etc/group

@@ -5,10 +5,18 @@ sys:x:3:
 adm:x:4:
 tty:x:5:
 disk:x:6:
+lp:x:7:
+kmem:x:9:
 wheel:x:10:root
+cdrom:x:11:
+dialout:x:18:
+floppy:x:19:
+video:x:28:
 audio:x:29:
+tape:x:32:
 www-data:x:33:
 utmp:x:43:
+plugdev:x:46:
 staff:x:50:
 lock:x:54:
 haldaemon:x:68: