net: ping: make do_ping() available via <net.h>

Make the do_ping() function in cmd/net.c a global one by getting rid of
the static qualifier, and move the prototype declaration from net-lwip.h
to net-common.h. This makes the function available to other parts of
U-Boot when CONFIG_NET=y, as was already the case when
CONFIG_NET_LWIP=y.

This is a peparation step to make the sandbox tests use a common API
between NET and NET_LWIP.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/net-common.h

@@ -493,6 +493,18 @@ int net_loop(enum proto_t protocol);
  */
 int dhcp_run(ulong addr, const char *fname, bool autoload);
 
+
+/**
+ * do_ping - Run the ping command
+ *
+ * @cmdtp: Unused
+ * @flag: Command flags (CMD_FLAG_...)
+ * @argc: Number of arguments
+ * @argv: List of arguments
+ * Return: result (see enum command_ret_t)
+ */
+int do_ping(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[]);
+
 /**
  * do_tftpb - Run the tftpboot command
  *