binman: Put fake files in a subdirectory

At present fake files from a previous build appear to be real files for
a subsequent build, since they sit in the output directory.

This can cause problems, since binman may need to parse the file, e.g.
with the Intel description.bin files.

Fix this by putting them in a 'binman-fake' subdirectory. Keep a track
of the fake filename so we only create it once. Subsequent builds will
still see that the file is missing and mark it as fake.

Update a few tests to check the behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index e3767ae..28a7510 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -9,6 +9,7 @@
 import os
 import pathlib
 import sys
+import time
 
 from binman import bintool
 from binman import comp_util
@@ -82,7 +83,10 @@
         missing_bintools: List of missing bintools for this entry
         update_hash: True if this entry's "hash" subnode should be
             updated with a hash of the entry contents
+        fake_fname: Fake filename, if one was created, else None
     """
+    fake_dir = None
+
     def __init__(self, section, etype, node, name_prefix=''):
         # Put this here to allow entry-docs and help to work without libfdt
         global state
@@ -116,6 +120,7 @@
         self.bintools = {}
         self.missing_bintools = []
         self.update_hash = True
+        self.fake_fname = None
 
     @staticmethod
     def FindEntryClass(etype, expanded):
@@ -1014,12 +1019,14 @@
                 bool: True if the blob was faked, False if not
         """
         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(size)
+            if not self.fake_fname:
+                outfname = os.path.join(self.fake_dir, os.path.basename(fname))
+                with open(outfname, "wb") as out:
+                    out.truncate(size)
+                tout.info(f"Entry '{self._node.path}': Faked blob '{outfname}'")
+                self.fake_fname = outfname
             self.faked = True
-            tout.info(f"Entry '{self._node.path}': Faked file '{outfname}'")
-            return outfname, True
+            return self.fake_fname, True
         return fname, False
 
     def CheckFakedBlobs(self, faked_blobs_list):
@@ -1169,3 +1176,11 @@
         fname = tools.get_output_filename(f'{prefix}.{uniq}')
         tools.write_file(fname, data)
         return data, fname, uniq
+
+    @classmethod
+    def create_fake_dir(cls):
+        """Create the directory for fake files"""
+        cls.fake_dir = tools.get_output_filename('binman-fake')
+        if not os.path.exists(cls.fake_dir):
+            os.mkdir(cls.fake_dir)
+        tout.notice(f"Fake-blob dir is '{cls.fake_dir}'")