@Werner Thank you for the reply.
Unfortunately, I have little knowledge about building.
However, I did find the code for rock-5c(rock-5c.conf), it's extremely shorter than the rock-5b. It has only 27 lines. Of course there are no function post_config_uboot_target__extra_configs_for_rock5c_mainline_environment_in_spi()
It means rock-5c.conf has not finished yet.
Can you guys take a look?