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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>