depends.py: Do not fail when disabling a non-existing option

To ease the removal of legacy crypto options, do not
fail in depends.py when disabling a non-existing option.
This mimics the behavior of 'config.py unset'.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/scripts/depends.py b/tests/scripts/depends.py
index 0cb5537..08829d1 100755
--- a/tests/scripts/depends.py
+++ b/tests/scripts/depends.py
@@ -109,6 +109,8 @@
 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:
+            return True
         log_line('Symbol {} was not found in {}'.format(option, conf.filename), color=colors.red)
         return False