package/busybox/udhcpc.script: support RFC3442 static routes

RFC3442 specifies a DHCP extension to provide the client with a list of
static routes to use.  This is already handled by udhcpc and exposed as the
"staticroutes" environment variable, but currently not handled by the action
script.

Extend the script to do so.  The RFC specifies that if this option is
provided by the server then the normal "routes" (3) option should be
ignored, so ensure that is done.

As we may now have more than just a default route on the interface, extend
the route cleanup logic to handle all routes for the interface (except for
the implied local 0.0.0.0 one).

Notice that this option is only sent by servers if explicitly requested by
the client, E.G.  using the -O staticroutes option to udhcpc.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>