[netdevice] Allow MTU to be changed at runtime

Provide a settings applicator to modify netdev->max_pkt_len in
response to changes to the "mtu" setting (DHCP option 26).

Note that as with MAC address changes, drivers are permitted to
completely ignore any changes in the MTU value.  The net result will
be that iPXE effectively uses the smaller of either the hardware
default MTU or the software configured MTU.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/dhcp.h

@@ -83,6 +83,9 @@ struct dhcp_packet;
 /** Root path */
 #define DHCP_ROOT_PATH 17
 
+/** Maximum transmission unit */
+#define DHCP_MTU 26
+
 /** Vendor encapsulated options */
 #define DHCP_VENDOR_ENCAP 43