mkfs.ubifs: Allow root entry in device table

mkfs.ubifs: Allow root entry in device table

When using a local root folder the permissions, user and group settings
are taken from the local folder. These permissions might be incorrect if
the folder has been created for the local user. Creating an UBIFS image
on my local system resulted in the following output on the target:

drwx------   17 1000     1000   1264 Jan  1 00:00 .
drwx------   17 1000     1000   1264 Jan  1 00:00 ..
drwxr-xr-x    2 root     root   9104 May 30  2017 bin
drwxr-xr-x    7 root     root   2760 Jan  1 00:00 dev
...

mkfs.ubifs aborts with an error message when the device table contains
a root entry. This patch allows setting the root folder permissions,
user and group to overwrite local configurations.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>