U-Boot setup a few variables :
- fdt_addr which is the board static dtb (most of the time loaded before u-boot or coming from some hardware like a ROM)
- fdt_addr_r which is a location in RAM that holds the DTB loaded by u-boot or before u-boot
In the case of u-boot + rpi firmware the DTB is loaded in RAM but the location
still end up in the fdt_addr variable and the fdt_addr_r variable exist.
Change the behavior so we test that a DTB exists for every possible variable :
- fdt_addr_r is checked first as if u-boot needed to modify it the correct DTB will live there.
- fdt_addr is checked second as if we run on a hardware with DTB in ROM it means that we what/need to run that
- fdtaddr looks like a FreeBSD-ism but since I'm not sure leave it.