How does efiboot know which DTB to load?

[Lloyd Parkes <lloyd%must-have-coffee.gen.nz@localhost>]

Kia ora koutou,
I went and added a new device tree file to my local sources for my
AX7021B board and build a new image. It seems to work. This is
unexpected. 

How does efiboot know to load my new dtb file? It does seem to be my
new file that is loaded and I did invent a top level "compatible" that
looked like it might be useful, but how does efiboot know what the FAT
filename is? 

My DTB is not the one that U-Boot is compiled with.

Ngā migi,
Lloyd