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>
diff --git a/net/Kconfig b/net/Kconfig
index 1b3e420..c4b4dae 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -74,4 +74,18 @@
 	  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