board: sl28: disable random MAC address generation

Nowadays, u-boot (when CONFIG_NET_RANDOM_ETHADDR is set) will set
enetaddr to a random value if not set and then pass the randomly
generated MAC address to linux.

This is bad for the following reasons:
 (1) it makes it impossible for linux to detect this error
 (2) linux won't trigger any fallback mechanism for the case where
     it didn't find any valid MAC address
 (3) a saveenv will store this randomly generated MAC address in the
     environment

Probably, the user will also be unaware that something is wrong. He will
just get different MAC addresses on each reboot, asking himself why this
is the case.

As this board usually have a serial port, the user can just fix this by
setting the MAC address manually in the environment. Also disable the
netconsole just in case, because it cannot be guaranteed that it will
work in any case. After all, this was just a convenience option, because
the bootloader - right now - doesn't have the ability to read the MAC
address, which is stored in the OTP. But it is far more important to
have a clear view of whats wrong with a board and that means we can no
longer use this Kconfig option.

Signed-off-by: Michael Walle <michael@walle.cc>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>