globalvar: Do not modify already existing variables

When globalvar_add_simple was called on previously existing variables
then the value was overwritten, even when value was passed as NULL.
Do not overwrite the value in this case which allows us to do a 'global'
on the same variable multiple times without loosing the values.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

common/globalvar.c

@@ -72,6 +72,9 @@ int globalvar_add_simple(const char *name, const char *value)
 	if (ret && ret != -EEXIST)
 		return ret;
 
+	if (!value)
+		return 0;
+
 	return dev_set_param(&global_device, name, value);
 }