netifd: update dhcp.script to handle dynamic routing

netifd: update dhcp.script to handle dynamic routing

Certain DHCP servers push a gateway outside of the assigned interface subnet,
to support those situations, install a host route towards the gateway.

If Gateway and IP are served in same network, openwrt quagga cannot learn
routes (rip routes are not getting added, showing inactive) whereas
working fine when Gateway and IP are in different network.

Signed-off-by: Mogula Pranay <mogula.pranay@nxp.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>