net: dhcp6: Add DHCPv6 (DHCP for IPv6)

Adds DHCPv6 protocol to u-boot.

Allows for address assignement with DHCPv6 4-message exchange
(SOLICIT->ADVERTISE->REQUEST->REPLY).  Includes DHCPv6 options
required by RFC 8415.  Also adds DHCPv6 options required
for PXE boot.

Possible enhancements:
- Duplicate address detection on DHCPv6 assigned address
- IPv6 address assignement through SLAAC
- Sending/parsing other DHCPv6 options (NTP, DNS, etc...)

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/include/net.h b/include/net.h
index 399af5e..181a6e3 100644
--- a/include/net.h
+++ b/include/net.h
@@ -484,6 +484,8 @@
 #ifdef CONFIG_NET
 extern char	net_root_path[CONFIG_BOOTP_MAX_ROOT_PATH_LEN];	/* Our root path */
 #endif
+/* Indicates whether the pxe path prefix / config file was specified in dhcp option */
+extern char *pxelinux_configfile;
 /** END OF BOOTP EXTENTIONS **/
 extern u8		net_ethaddr[ARP_HLEN];		/* Our ethernet address */
 extern u8		net_server_ethaddr[ARP_HLEN];	/* Boot server enet address */
@@ -504,8 +506,8 @@
 extern int		net_restart_wrap;	/* Tried all network devices */
 
 enum proto_t {
-	BOOTP, RARP, ARP, TFTPGET, DHCP, PING, PING6, DNS, NFS, CDP, NETCONS,
-	SNTP, TFTPSRV, TFTPPUT, LINKLOCAL, FASTBOOT, WOL, UDP, NCSI, WGET
+	BOOTP, RARP, ARP, TFTPGET, DHCP, DHCP6, PING, PING6, DNS, NFS, CDP,
+	NETCONS, SNTP, TFTPSRV, TFTPPUT, LINKLOCAL, FASTBOOT, WOL, UDP, NCSI, WGET
 };
 
 extern char	net_boot_file_name[1024];/* Boot File name */