Ignore non-UTF8 chars in UF2 upload (#148)

Fixes #95

tools/uf2conv.py

@@ -222,7 +222,7 @@ def convert_from_hex_to_uf2(buf):
     return resfile
 
 def to_str(b):
-    return b.decode("utf-8")
+    return b.decode("utf-8", "replace")
 
 def get_drives():
     drives = []