udev-extraconf: Allow FAT mount group to be specified

udev-extraconf: Allow FAT mount group to be specified

Adds a variable that allows the group given permissions to access FAT
mounted file systems to be specified, instead of being hardcoded to
'disk'.

As a note, the usage of 'disk' as the mount group is not a very secure
default, since the disk user usually has read/write access to all block
devices in /dev/, meaning that adding a user to this group for the
purpose of accessing FAT file systems effectively gives the write access
to all block devices.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>