configure: add configure option to disable building brcompat

This adds ability to do:

  ./configure --disable-brcompat

to disable building userspace and kernel module associated with
providing linux bridge compatibility.  Sources should still be
distributed w/ make dist.

While there, update comment referring to long removed veth driver
which is now relevant for brcompat module.

Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>