cmd: bootvx: Pass netmask and gatewayip to VxWorks bootline

There are fields in VxWorks bootline for netmask and gatewayip.
We can get these from U-Boot environment variables and pass them
to VxWorks, just like ipaddr and serverip.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>