binman: Support reading an image with entry args

Normally when an entry is created, any entry arguments it has are required
to be provided, so it can actually generate its contents correctly.

However when an existing image is read, Entry objects are created for each
of the entries in the image. This happens as part of the process of
reading the image into binman.

In this case we don't need the entry arguments, since we do not intend to
regenerate the entries, or at least not unless requested. So there is no
sense in reporting an error for missing entry arguments.

Add a new property for the Image to handle this case. Update the error
reporting to be conditional on this property.

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

tools/binman/image.py

@@ -43,8 +43,13 @@ class Image(section.Entry_section):
         test: True if this is being called from a test of Images. This this case
             there is no device tree defining the structure of the section, so
             we create a section manually.
+        ignore_missing: Ignore any missing entry arguments (i.e. don't raise an
+            exception). This should be used if the Image is being loaded from
+            a file rather than generated. In that case we obviously don't need
+            the entry arguments since the contents already exists.
     """
-    def __init__(self, name, node, copy_to_orig=True, test=False):
+    def __init__(self, name, node, copy_to_orig=True, test=False,
+                 ignore_missing=False):
         super().__init__(None, 'section', node, test=test)
         self.copy_to_orig = copy_to_orig
         self.name = 'main-section'
@@ -53,6 +58,7 @@ class Image(section.Entry_section):
         self.fdtmap_dtb = None
         self.fdtmap_data = None
         self.allow_repack = False
+        self._ignore_missing = ignore_missing
         if not test:
             self.ReadNode()
 
@@ -100,7 +106,7 @@ class Image(section.Entry_section):
 
         # Return an Image with the associated nodes
         root = dtb.GetRoot()
-        image = Image('image', root, copy_to_orig=False)
+        image = Image('image', root, copy_to_orig=False, ignore_missing=True)
 
         image.image_node = fdt_util.GetString(root, 'image-node', 'image')
         image.fdtmap_dtb = dtb