Blackfin: drop MAC display at boot

The default Blackfin boot would display the MAC address for the first NIC,
but this relies on the environment.  The current net multi stack no longer
writes the default hardware settings to the environment, so most of the
time the display shows all zeros.  This can be pretty confusing and really
doesn't add anything useful, so just drop it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>