esp8266/makeimg.py: Append md5 hash to the generated binary.

md5 is calculated over the entire file, except first 4 bytes, which contain
flash parameters and may be changed by flashing tool or MicroPython flash
auto-config.

esp8266/makeimg.py

 import sys
 import struct
+import hashlib
 
 SEGS_MAX_SIZE = 0x9000
 
 assert len(sys.argv) == 4
 
+md5 = hashlib.md5()
+
 with open(sys.argv[3], 'wb') as fout:
 
     with open(sys.argv[1], 'rb') as f:
         data_flash = f.read()
         fout.write(data_flash)
+        # First 4 bytes include flash size, etc. which may be changed
+        # by esptool.py, etc.
+        md5.update(data_flash[4:])
         print('flash    ', len(data_flash))
 
     with open(sys.argv[2], 'rb') as f:
@@ -18,10 +24,17 @@ with open(sys.argv[3], 'wb') as fout:
     pad = b'\xff' * (SEGS_MAX_SIZE - len(data_flash))
     assert len(pad) >= 4
     fout.write(pad[:-4])
-    fout.write(struct.pack("I", SEGS_MAX_SIZE + len(data_rom)))
+    md5.update(pad[:-4])
+    len_data = struct.pack("I", SEGS_MAX_SIZE + len(data_rom))
+    fout.write(len_data)
+    md5.update(len_data)
     print('padding  ', len(pad))
 
     fout.write(data_rom)
+    md5.update(data_rom)
     print('irom0text', len(data_rom))
 
+    fout.write(md5.digest())
+
     print('total    ', SEGS_MAX_SIZE + len(data_rom))
+    print('md5      ', md5.hexdigest())