Adds basic support for ProxyDHCP

- ProxyDHCP allows a second DHCP server to exist alongside your main
  DHCP server and supply additional BOOTP related options
- When u-boot sends out a DHCP request, the real DHCP server will
  respond with a normal response containing the new client IP address
  while simultaneously the ProxyDHCP server will respond with a blank
  client IP address and a `bootfile` option
- This patch adds CONFIG_SERVERIP_FROM_PROXYDHCP (default false) to
  enable this behavior and CONFIG_SERVERIP_FROM_PROXYDHCP_DELAY_MS
  (default 100) which tells u-boot to wait additional time after
  receiving the main DHCP response to give the ProxyDHCP response time
  to arrive
- The PXE spec for ProxyDHCP is more complicated than the solution
  added here as diagramed on page 16:
  http://www.pix.net/software/pxeboot/archive/pxespec.pdf:

```
DHCP Discover will be retried four times. The four timeouts are 4, 8, 16
and 32 seconds respectively. If a DHCPOFFER is received without an Option
timeouts in an attempt to receive a PXE response.
```

- Adding a simple delay worked for my purposes but let me know if a
  more robust solution is required

Signed-off-by: Lyle Franklin <lylejfranklin@gmail.com>

net/Kconfig

@@ -74,4 +74,18 @@ config TFTP_WINDOWSIZE
 	  before an ack response is required.
 	  The default TFTP implementation implies a window size of 1.
 
+config SERVERIP_FROM_PROXYDHCP
+	bool "Get serverip value from Proxy DHCP response"
+	help
+	  Allows bootfile config to be fetched from Proxy DHCP server
+		while IP is obtained from main DHCP server.
+
+config SERVERIP_FROM_PROXYDHCP_DELAY_MS
+	int "# of additional milliseconds to wait for ProxyDHCP response"
+	default 100
+	help
+	  Amount of additional time to wait for ProxyDHCP response after
+		receiving response from main DHCP server. Has no effect if
+		SERVERIP_FROM_PROXYDHCP is false.
+
 endif   # if NET

net/bootp.c

@@ -146,10 +146,7 @@ static int check_reply_packet(uchar *pkt, unsigned dest, unsigned src,
 	return retval;
 }
 
-/*
- * Copy parameters of interest from BOOTP_REPLY/DHCP_OFFER packet
- */
-static void store_net_params(struct bootp_hdr *bp)
+static void store_bootp_params(struct bootp_hdr *bp)
 {
 #if !defined(CONFIG_BOOTP_SERVERIP)
 	struct in_addr tmp_ip;
@@ -182,6 +179,16 @@ static void store_net_params(struct bootp_hdr *bp)
 	 */
 	if (*net_boot_file_name)
 		env_set("bootfile", net_boot_file_name);
+#endif
+}
+
+/*
+ * Copy parameters of interest from BOOTP_REPLY/DHCP_OFFER packet
+ */
+static void store_net_params(struct bootp_hdr *bp)
+{
+#if !defined(CONFIG_SERVERIP_FROM_PROXYDHCP)
+	store_bootp_params(bp);
 #endif
 	net_copy_ip(&net_ip, &bp->bp_yiaddr);
 }
@@ -1055,8 +1062,12 @@ static void dhcp_handler(uchar *pkt, unsigned dest, struct in_addr sip,
 	debug("DHCPHandler: got DHCP packet: (src=%d, dst=%d, len=%d) state: "
 	      "%d\n", src, dest, len, dhcp_state);
 
-	if (net_read_ip(&bp->bp_yiaddr).s_addr == 0)
+	if (net_read_ip(&bp->bp_yiaddr).s_addr == 0) {
+#if defined(CONFIG_SERVERIP_FROM_PROXYDHCP)
+		store_bootp_params(bp);
+#endif
 		return;
+	}
 
 	switch (dhcp_state) {
 	case SELECTING:
@@ -1075,6 +1086,12 @@ static void dhcp_handler(uchar *pkt, unsigned dest, struct in_addr sip,
 			dhcp_packet_process_options(bp);
 			efi_net_set_dhcp_ack(pkt, len);
 
+#if defined(CONFIG_SERVERIP_FROM_PROXYDHCP)
+			if (!net_server_ip.s_addr)
+				udelay(CONFIG_SERVERIP_FROM_PROXYDHCP_DELAY_MS *
+					1000);
+#endif	/* CONFIG_SERVERIP_FROM_PROXYDHCP */
+
 			debug("TRANSITIONING TO REQUESTING STATE\n");
 			dhcp_state = REQUESTING;