esp8266/makeimg.py: Store firmware size as last 4 bytes of padding area.

esp8266/makeimg.py

 import sys
+import struct
 
 SEGS_MAX_SIZE = 0x9000
 
@@ -11,13 +12,16 @@ with open(sys.argv[3], 'wb') as fout:
         fout.write(data_flash)
         print('flash    ', len(data_flash))
 
+    with open(sys.argv[2], 'rb') as f:
+        data_rom = f.read()
+
     pad = b'\xff' * (SEGS_MAX_SIZE - len(data_flash))
-    fout.write(pad)
+    assert len(pad) >= 4
+    fout.write(pad[:-4])
+    fout.write(struct.pack("I", SEGS_MAX_SIZE + len(data_rom)))
     print('padding  ', len(pad))
 
-    with open(sys.argv[2], 'rb') as f:
-        data_rom = f.read()
-        fout.write(data_rom)
-        print('irom0text', len(data_rom))
+    fout.write(data_rom)
+    print('irom0text', len(data_rom))
 
     print('total    ', SEGS_MAX_SIZE + len(data_rom))