The on-board VPD EEPROM isn't write protected. In fact, it cannot be write protected because the boot counter is stored there. If you accidentially deleted your u-boot environment and you don't have a valid VPD content, the serial number and the base MAC address is still stored in the OTP region of the SPI flash. You can use this tool to read the OTP data.

$ gcc -Wall -std=c99 -o otpvpd otpvpd.c

You can find a sample script restore-u-boot-variables in the contrib directory. This will use the u-boot-envtools to set the variables.

⚠️ The u-boot-envtools prior to version 2020.10 will always try to lock the environment sector. You might not be able to modify it within u-boot anymore.