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/svf/svf.c

@@ -661,11 +661,13 @@ static int svf_read_command_from_file(FILE *fd)
 				if (svf_getline(&svf_read_line, &svf_read_line_size, svf_fd) <= 0)
 					return ERROR_FAIL;
 				i = -1;
+				/* fallthrough */
 			case '\r':
 				slash = 0;
 				/* Don't save '\r' and '\n' if no data is parsed */
 				if (!cmd_pos)
 					break;
+				/* fallthrough */
 			default:
 				/* The parsing code currently expects a space
 				 * before parentheses -- "TDI (123)".  Also a