lava/renesas: use the U-Boot for fdtfile setup

Starting from kernel 5.5 FDT names were split and
renamed to align with config options. This leads to
boot failure of old or new Apertis images depending of
naming schema used in `fdtfile` environment variable.

U-Boot used in Apertis and LAVA have additional feature
allowing to try the legacy FDT naming schema in cases
of boot failure with default naming schema.

On v2021 for the switching branches test we have to boot
images with old naming for DTB file in v2020 (5.4 kernel) and
new naming schema in v2021 (5.9 kernel).
So it is not possible to hardcode proper variable `fdtfile`
for booting both versions in LAVA.

Instead we remove hardcoded DTB file and use additional boot
target `legacy_mmc0` which automatically set the "old" name
for the DTB file in case if boot with a new one was failed.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>