v2021: 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