Rotate the framebuffer during boot in Linux
If your kernel is compiled with CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y which it probably is if you’re using CentOS, Fedora, Ubuntu, or any of the other mainstream distributions, then you can rotate the framebuffer console output to match your rotated monitors.
You can also rotate the current framebuffer console from a booted system like this:
echo 3 >> /sys/class/graphics/fbcon/rotate
If the text is upside down, try rotate:1 instead. The rotation should take effect immediately. If the rotation worked then you can make it permanent by adding it to the kernel line during boot.
Add this to your kernel line in grub: