Custom stations: Revisit checksum generation

Some AVRs do not allow an ID larger than 15 characters. Thanks to Marc for pointing that out.
Use a MD5 hash (128), XOR fold it (64) and cut it to size (48). This way we _should_ still have enough uniqueness and can generate a checksum which fits the limits.

ycast/my_stations.py

@@ -75,7 +75,12 @@ def get_stations_by_category(category):
     return stations
 
 
-def get_checksum(feed):
+def get_checksum(feed, charlimit=12):
     hash_feed = feed.encode()
-    hash_object = hashlib.sha256(hash_feed)
-    return hash_object.hexdigest()
+    hash_object = hashlib.md5(hash_feed)
+    digest = hash_object.digest()
+    xor_fold = bytearray(digest[:8])
+    for i, b in enumerate(digest[8:]):
+        xor_fold[i] ^= b
+    digest_xor_fold = ''.join(format(x, '02x') for x in bytes(xor_fold))
+    return digest_xor_fold[:charlimit]