lava: Fix NFS tests on Renesas boards

After commit 45dae71b fixed the boot on Renesas boards with linux >= 5.7 due to the change of the dtb name, the tests exercising the NFS boot of the current release as the first stage broke since they do not use boot_commands and thus didn't get the override.

To fix that, rather than constructing the path to the DTB from the LAVA device name, which no longer match the board type with linux >= 5.7, explicitly pass the DTB path for the first stage and the release under test separately.

Forward port of !196 (merged).