ipconfig: add carrier_timeout kernel parameter

commit 3fb72f1e ("ipconfig wait for carrier") added a
"wait for carrier" policy, with a fixed worst case maximum wait
of two minutes.

Now make the wait for carrier timeout configurable on the kernel
commandline and use the 120s as the default.

The timeout messages introduced with
commit 5e404cd6 ("ipconfig: add informative timeout messages while
waiting for carrier") are done in a fixed interval of 20 seconds, just
like they were before (240/12).

Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>