UEFI fallback boot mode
The UEFI specification defines a so called "fallback boot mode" which allows booting an efi file from disks or removable disks (e.g. USB sticks)
without a corresponding boot variable existing in NVRAM.
When the UEFI firmware is directed to boot from a disk it first searches for a EFI System partition (on GPT disks)
or a FAT32 partition (on MBR disks).
Subsequently, if such a partition is found the firmware searches for a file
(on x86-64 machines), and if found, starts this efi file.