Integrate uboot-imx-kuk/imx8_temp_grade_detection

Commit: seco-ne/3rd-party/kuk/uboot-imx-kuk@5b7e1241

board:common: Temp grade detection

Add a common function to export the current temperature grade as environment var. The var should later be evaluated by bootscript to set the appropriate temperature limits in the devicetree. Enabled the temp grade export for imx8m platforms.

BCS 746-001385