net: Fix potential empty DHCP Parameter Request List

Can't get IP address with dhcp due to the dhcp server not
allow the empty param list request under some network env

This patch is based on Gray Remlin's initial patch.
Signed-off-by: NJason Liu <r64343@freescale.com>
Signed-off-by: NGray Remlin <g_remlin@rocketmail.com>

net/bootp.c

@@ -456,6 +456,10 @@ static int DhcpExtended (u8 * e, int message_type, IPaddr_t ServerID, IPaddr_t R
 	*e++  = 42;
 	*cnt += 1;
 #endif
+	/* no options, so back up to avoid sending an empty request list */
+	if (*cnt == 0)
+		e -= 2;
+
 	*e++  = 255;		/* End of the list */
 
 	/* Pad to minimal length */