uip: build the forwarding module only when configured

uip-fw.c forwards packets between several network interfaces. Nothing in
this firmware calls it: the only references to uip_fw_output() outside the
module are commented out in uip-split.c. It is compiled and linked all the
same, and its register pressure lands in direct RAM, which on SWTGW218AS is
already reported as having no spare space.

Add UIP_CONF_FORWARDING alongside the existing UIP_CONF_BROADCAST and build
the module only when it is set. A device that forwards turns it on and gets
the module unchanged; a single-interface device stops paying for it.

On SWTGW218AS the stack gains 9 bytes, from 151 available to 160, and BANK1
drops 1200 bytes, from 47994 to 46794.

uip/uip-fw.c

@@ -64,6 +64,8 @@
 
 #include "../rtl837x_common.h"
 
+#if UIP_FORWARDING
+
 /*
  * The list of registered network interfaces.
  */
@@ -535,3 +537,5 @@ uip_fw_periodic(void)
   }
 }
 /*------------------------------------------------------------------------------*/
+
+#endif /* UIP_FORWARDING */