reproducing steps:

1. Define a host group A where you set OS and configure "PXELinux BIOS" (the value does not matter)
2. Define sub host group B, set A as a parent
3. The PXE loader attribute does not inherit anything and prints Inherited parent (no value)

it should show "Inherited value (PXELinux BIOS)" but it does not work. The reason is that other attributes respecting inheritance combines two methods. For example `ptable_id` only return uninherited value while `ptable` works with inheritace. Since pxe_loader is not relation but simple attribute this does not work.

I started to work on the fix but didn't have time to finish it, feel free to inspire at here, the host group form works, but when it's applied in host form, the pxe_loader remains blank.

A workaround I had to do was assigning the pxeloader in all child host groups.

Present probably since 1.13+, see #12634