tools/mpy-tool.py: Fix frozen comment generation to escape chars.

That caused the compile of frozen_content.c to fail if characters like
backslash were in a short string.  Thanks to @hippy for identifying the
spot to change.

tools/mpy-tool.py

@@ -861,7 +861,7 @@ def freeze(self):
             fmt, sz, arg = mp_opcode_decode(bc, ip)
             opcode_name = Opcodes.mapping[bc[ip]]
             if fmt == MP_BC_FORMAT_QSTR:
-                opcode_name += " " + self.qstr_table[arg].str
+                opcode_name += " " + repr(self.qstr_table[arg].str)
             elif fmt in (MP_BC_FORMAT_VAR_UINT, MP_BC_FORMAT_OFFSET):
                 opcode_name += " %u" % arg
             print(