binman: Make fake blobs zero-sized by default

On x86 devices having even a small amount of data can cause an overlap
between regions. For example, bayleybay complains when the intel-vga
region overlaps with u-boot-ucode:

   ImagePos    Offset      Size  Name
   <none>    00000000  00800000  main-section
   <none>     ff800000  00000080  intel-descriptor
   <none>     ff800400  00000080  intel-me
   <none>     fff00000  00098f24  u-boot-with-ucode-ptr
   <none>     fff98f24  00001aa0  u-boot-dtb-with-ucode
   <none>     fff9a9d0  0002a000  u-boot-ucode
   <none>     fffb0000  00000080  intel-vga
   ...

It is safer to use an empty file in most cases. Add an option to set the
size for those uses that need it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

tools/binman/entry.py

@@ -990,13 +990,14 @@ features to produce new behaviours.
         if self.missing:
             missing_list.append(self)
 
-    def check_fake_fname(self, fname):
+    def check_fake_fname(self, fname, size=0):
         """If the file is missing and the entry allows fake blobs, fake it
 
         Sets self.faked to True if faked
 
         Args:
             fname (str): Filename to check
+            size (int): Size of fake file to create
 
         Returns:
             tuple:
@@ -1006,7 +1007,7 @@ features to produce new behaviours.
         if self.allow_fake and not pathlib.Path(fname).is_file():
             outfname = tools.get_output_filename(os.path.basename(fname))
             with open(outfname, "wb") as out:
-                out.truncate(1024)
+                out.truncate(size)
             self.faked = True
             tout.info(f"Entry '{self._node.path}': Faked file '{outfname}'")
             return outfname, True