[EXPORT][CoIDE] problem with missing linker script

- a missing linker script configures the scatter file option (linker
script path in CoIDE) to 'None' not to ''

workspace_tools/export/coide.py

@@ -62,6 +62,9 @@ class CoIDE(Exporter):
             l, _ = splitext(basename(lib))
             libraries.append(l[3:])
 
+        if self.resources.linker_script is None:
+            self.resources.linker_script = ''
+            
         ctx = {
             'name': self.program_name,
             'source_files': source_files,