Some versions of U-Boot do not include a "media device path" node in
the boot device path, even though the OpenBSD bootloader has been
loaded from an MS-DOS partition.
The UEFI specification isn't very clear whether such a media device
path node is required or not. Unfortunately our current EFI
bootloader code pretty much requires it. The diff below fixes that by
having efi_device_path_depth() return the full depth if no media
device path node is found.
This makes OpenBSD boot reliable on the Theobroma Systems RK3399 Q7
"Puma" board. And it might help booting using some of the "default"
U-Boot setups on the rock64, rockpro64 and pinebook pro.
Tested this on armv7 (U-Boot), arm64 (U-Boot and real UEFI) and amd64