tools/env: allow overwrite of ethaddr on default

This patch allows the U-Boot user space companion utility, fw_setenv,
to overwrite the 'ethaddr' key/value pair if the current value is set
to a per-board-configured default.

This change allows 'fw_setenv' to match the behavior of 'setenv' /
'env set' on the U-Boot command line.

Signed-off-by: Grant Erickson <marathon96@gmail.com>
Fixed excessive white space.
Signed-off-by: Wolfgang Denk <wd@denx.de>