Fix GCC7 warnings about switch-case fallthroughs

GCC7 with -Wextra warns about switch-case blocks which fallthrough with
"this statement may fall through [-Werror=implicit-fallthrough=]". This
can be fixed by adding "special" comments: "/* fallthrough */".

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Iba0be791dbdd86984489b2d9a0592bb59828da1e
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4174
Tested-by: jenkins

src/helper/command.c

@@ -1456,8 +1456,8 @@ COMMAND_HELPER(handle_command_parse_bool, bool *out, const char *label)
 				LOG_ERROR("%s: argument '%s' is not valid", CMD_NAME, in);
 				return ERROR_COMMAND_SYNTAX_ERROR;
 			}
-			/* fall through */
 		}
+			/* fallthrough */
 		case 0:
 			LOG_INFO("%s is %s", label, *out ? "enabled" : "disabled");
 			break;