Previously the EthernetInterface class was unable to be connected
after being brought down. The core issue is that lwip was not designed
to be completely brought down due once initialized.
To work around this, the ethernet interface only initializes once and
leaves itself up after a disconnect call. The DHCP lease is still
released/acquired on disconnect/connect.
Having the echo tests generate any random character
caused issues serializing the test output into an
xml file. This change limits the characters generated
by the device to '0' - '9'.
Sam Grove
Christopher Haster
Brian Daniels
Russ Butler