About this question, here is a unified reply.
We need to see this place where one is the definition and load the configuration.
https://github.com/sipeed/MaixPy/blob/master/components/micropython/port/builtin_py/board.py
If you don't provide the configuration, you won't get the concrete variable.
If it is SIPEED published hardware, the appropriate configuration is provided here.
htt
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OS: Ubuntu 18.04.02 LTS
The error occures because on line 86 rsync fails to expand the asterisk. Works as intended on Debian 10.