dhcpcd: Send unique IAID

The current logic uses the kernel iface index when the name is longer
than 4 octets.  This is not stable -- the iface index changes under
various scenarios, such as toggling airplane mode.

Instead, use the lower 4 octets of the iface hwaddr.  This is the same
logic used in the current dhcpcd code and in the ISC DHCP client.

Note the upstream fix is here:

  http://roy.marples.name/projects/dhcpcd/ci/31ea92e995477728?sbs=1

This is several revisions beyond our current code and it is rumored
that the next major AOSP release will switch to a different DHCP
client, so we won't bother trying to sync up with upstream.

Change-Id: I0663b9bcf3803978adcef6d936ae49e128c7b1f2

dhcpcd.c

@@ -776,7 +776,7 @@ configure_interface1(struct interface *iface)
 	struct if_state *ifs = iface->state;
 	struct if_options *ifo = ifs->options;
 	uint8_t *duid;
-	size_t len = 0, ifl;
+	size_t len = 0;
 
 	/* Do any platform specific configuration */
 	if_conf(iface);
@@ -824,16 +824,8 @@ configure_interface1(struct interface *iface)
 			iface->clientid = xmalloc(len + 6);
 			iface->clientid[0] = len + 5;
 			iface->clientid[1] = 255; /* RFC 4361 */
-			ifl = strlen(iface->name);
-			if (ifl < 5) {
-				memcpy(iface->clientid + 2, iface->name, ifl);
-				if (ifl < 4)
-					memset(iface->clientid + 2 + ifl,
-					    0, 4 - ifl);
-			} else {
-				ifl = htonl(if_nametoindex(iface->name));
-				memcpy(iface->clientid + 2, &ifl, 4);
-			}
+			memcpy(iface->clientid + 2,
+			    iface->hwaddr + iface->hwlen - 4, 4);
 			memcpy(iface->clientid + 6, duid, len);
 		} else if (len == 0) {
 			len = iface->hwlen + 1;