Re-instate failure when disabling non-existing option

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>

tests/scripts/depends.py

@@ -109,12 +109,6 @@ def set_config_option_value(conf, option, colors, value: Union[bool, str]):
 value can be either True/False (set/unset config option), or a string,
 which will make a symbol defined with a certain value."""
     if not option_exists(conf, option):
-        if value is False:
-            log_line(
-                f'Warning, disabling {option} that does not exist in {conf.filename}',
-                color=colors.cyan
-            )
-            return True
         log_line('Symbol {} was not found in {}'.format(option, conf.filename), color=colors.red)
         return False