uboot-imx

iot_dk: Disable networking support

There's no Ethernet controller on the board so no point in having
networking support. This also saves us 5.5 kB of precious memory.

| # bloat-o-meter u-boot.net u-boot.no_net_regex | tail -1
| Total: Before=127892, After=122334, chg -4.35%

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>