binman: Tidy up setting of entry contents

At present the contents of an entry are set in subclasses simply by
assigning to the data and content_size properties. Add some methods to do
this, so that we have more control. In particular, add a method to set the
contents without changing its size, so we can validate that case.

Add a test case for trying to change the size when this is not allowed.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/ftest.py

@@ -1006,5 +1006,13 @@ class TestFunctional(unittest.TestCase):
                 "processing of contents: remaining [<_testing.Entry__testing ",
                 str(e.exception))
 
+    def testBadChangeSize(self):
+        """Test that trying to change the size of an entry fails"""
+        with self.assertRaises(ValueError) as e:
+            self._DoReadFile('59_change_size.dts', True)
+        self.assertIn("Node '/binman/_testing': Cannot update entry size from "
+                      '2 to 1', str(e.exception))
+
+
 if __name__ == "__main__":
     unittest.main()