Add DNS support

On 04 Oct 2008 Pieter posted a dns implementation for U-Boot.

http://www.mail-archive.com/u-boot-users@lists.sourceforge.net/msg10216.html
>
> DNS can be enabled by setting CFG_CMD_DNS. After performing a query,
> the serverip environment var is updated.
>
> Probably there are some cosmetic issues with the patch. Unfortunatly I
> do not have the time to correct these. So if anybody else likes DNS
> support in U-Boot and has the time, feel free to patch it in the main tree.

Here it is again - slightly modified & smaller:
  - update to 2009-06 (Pieter's patch was for U-Boot 1.2.0)
  - README.dns is added
  - syntax is changed (now takes a third option, the env var to store
    the result in)
  - add a random port() function in net.c
  - sort Makefile in ./net/Makefile
  - dns just returns unless a env var is given
  - run through checkpatch, and clean up style issues
  - remove packet from stack
  - cleaned up some comments
  - failure returns much faster (if server responds, don't wait for
    timeout)
  - use built in functions (memcpy) rather than byte copy.

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Signed-off-by: Pieter Voorthuijsen <pieter.voorthuijsen@prodrive.nl>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>

include/net.h

@@ -361,6 +361,11 @@ typedef enum { BOOTP, RARP, ARP, TFTP, DHCP, PING, DNS, NFS, CDP, NETCONS, SNTP
 /* from net/net.c */
 extern char	BootFile[128];			/* Boot File name		*/
 
+#if defined(CONFIG_CMD_DNS)
+extern char *NetDNSResolve;		/* The host to resolve  */
+extern char *NetDNSenvvar;		/* the env var to put the ip into */
+#endif
+
 #if defined(CONFIG_CMD_PING)
 extern IPaddr_t	NetPingIP;			/* the ip address to ping		*/
 #endif