Recompile u-boot/kernel to rotate screen at Boot - Debian 9 - Tinker OS 2.0.4

I need help from a Linux guru so that I can recompile my u-boot or kernel to change the orientation of my display from landscape (default) to portrait at boot up. Changing the orientation of my screen using xrandr in my .xsessionrc file rotates the orientation after bootup, but does not stay permanent and reverts back to landscape after powering off the screen or unplugging the screen - It needs to be permanent. I'm able to provide Debian u-boot and Debian Kernel for this work. It is for an ASUS Tinker Board running Tinker OS 2.0.4 (Debian 9). Thanks.