add CMD_NAME macro for command handlers

By introducing the CMD_NAME macro, this parameter may be integrated
as args[-1] in command.[ch], without touching any other call sites.

src/flash/flash.c

@@ -728,7 +728,7 @@ COMMAND_HANDLER(handle_flash_fill_command)
 	if (count == 0)
 		return ERROR_OK;
 
-	switch (cmd[4])
+	switch (CMD_NAME[4])
 	{
 	case 'w':
 		wordsize = 4;