I don’t need USB boot, I need to load my own GRUB which is located at /dev/nvme0n1p1 as /efi/EFI/GRUB/grubx64.efi.
But it seems that PureBoot ignores this file and just parses my GRUB options located at /dev/nvme0n1p2 as /boot/grub/grub.cfg. PureBoot has its own menu entries with various versions of Linux kernel and memtest but it ignores my GRUB_BACKGROUND.