binman: Generate preload header and sign data only once

To optimize preload generation, generate the header and signatures only
after all data has been collected in ProcessContentsUpdate(). This
avoids signing the data multiple times.

Since header_size is known upfront (from __init__), create a placeholder
in `ObtainContents()` to avoid an extra packing pass when
ProcessContentsUpdate() detects a size change.

This reduces unnecessary repacking and signing operations.

Signed-off-by: Paul HENRYS <paul.henrys_ext@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/binman/etype/pre_load.py

@@ -152,14 +152,11 @@ class Entry_pre_load(Entry_collection):
         return data + pad
 
     def ObtainContents(self):
-        """Obtain a placeholder for the header contents"""
-        # wait that the image is available
-        self.image = self.GetContents(False)
-        if self.image is None:
-            return False
-        self.SetContents(self._CreateHeader())
+        """Create a placeholder for the header"""
+        self.SetContents(tools.get_bytes(0, self.header_size))
         return True
 
     def ProcessContents(self):
+        self.image = self.GetContents(True)
         data = self._CreateHeader()
         return self.ProcessContentsUpdate(data)