net: ping6: Add ping6 command

Implement ping6 command to ping hosts using IPv6. It works the same way as
an ordinary ping command. There is no ICMP request so it is not possible
to ping our host. This patch adds options in Kconfig and Makefile to
build ping6 command.

Series-changes: 3
- Added structures and functions descriptions
- Added to ping6_receive() return value instead of void

Series-changes: 4
- Fixed structures and functions description style

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/net.h

@@ -560,8 +560,8 @@ extern ushort		net_native_vlan;	/* Our Native VLAN */
 extern int		net_restart_wrap;	/* Tried all network devices */
 
 enum proto_t {
-	BOOTP, RARP, ARP, TFTPGET, DHCP, PING, DNS, NFS, CDP, NETCONS, SNTP,
-	TFTPSRV, TFTPPUT, LINKLOCAL, FASTBOOT, WOL, UDP, NCSI, WGET
+	BOOTP, RARP, ARP, TFTPGET, DHCP, 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 */