By moving the Mustang box into a lab that had a gigibit connection, it immediately connected with the network. It's surprising that the network worked just fine on a 100Kb connection when running tftpboot and when installing Ubuntu, but not when running Ubuntu. This looks like a limitation in the Ubuntu 14.04 driver for aarch64, not with the Mustang box itself. It's just too bad that this limitation isn't documented anywhere, other than this bug report.
By moving the Mustang box into a lab that had a gigibit connection, it immediately connected with the network. It's surprising that the network worked just fine on a 100Kb connection when running tftpboot and when installing Ubuntu, but not when running Ubuntu. This looks like a limitation in the Ubuntu 14.04 driver for aarch64, not with the Mustang box itself. It's just too bad that this limitation isn't documented anywhere, other than this bug report.