commands: add WestCommand.has_manifest property

This is useful for making decisions based on whether we have a
manifest.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

src/west/commands.py

@@ -166,6 +166,12 @@ class WestCommand(ABC):
     # subclass leads to undefined results.
     #
 
+    @property
+    def has_manifest(self):
+        '''Property which is True if self.manifest is safe to access.
+        '''
+        return self._manifest is not None
+
     @property
     def manifest(self):
         '''Property for the manifest which was passed to run().