v2020: lava: Fix the name of the DTB file for kernel < 5.7

LAVA is failing to boot images since the default DTB file name changed in linux 5.7 and the U-Boot bootloader has changed its defaults to reflect that, which means that it now fails to find the DTB when using older images that provide the DTB in the previous location.

To avoid that, explicitly provide the DTB name.

See also !217 (merged)