Do not config Gateway when DHCP is enabled
Do not config Gateway when DHCP is enabled

When DHCP is enabled and the Gateway is in the network config,
systemd-networkd will try to set the gateway anyway, and if the gateway
could not be set, it fails with below error:

 systemd-networkd[459]: eth0: Could not set route: No route to host
 systemd-networkd[459]: eth0: Failed

The config should skip setting the gateway when the DHCP is enabled.

Tested: Verify the DHCP could be enabled from static config.

Fixes openbmc/phosphor-networkd#43

Signed-off-by: Lei YU <fc52a17c06c071ee7cd115c972235443e2107013@bytedance.com>
Change-Id: I6486284133003afbe5c657ef22c27d1994544f17

openbmc/phosphor-networkd,openbmc/phosphor-networkd