external/mambo: fix tcl startup code for mambo bogus net (repost)
external/mambo: fix tcl startup code for mambo bogus net (repost)

Repost of the same thing with Signed-off-by, and Acked-by from Michael Neuling.

This fixes a couple issues with external/mambo/skiboot.tcl so I can use the
mambo bogus net.
 * newer distros (ubuntu 18.04) allow tap device to have a user specified
   name instead of just tapN so we need to pass in a name not a number.
 * need some kind of default for net_mac, and need the mconfig for it
   to be set from an env var.

Thanks,
   Aaron

Acked-by: Michael Neuling <mikey at neuling.org>
Signed-off-by: Aaron Sawdey <sawdey at linux.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

stewart-ibm/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot,stewart-ibm/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,legoater/skiboot