uml: pcap devices should get MACs from command line

Allow a pcap device to be assigned a MAC on the command line.  They don't
really need one, but it is handy to be able to do when your distro assigns a
new ethernet device whenever it sees a new MAC.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>