lava: fix the name of DTB file for 18.12 boot (backport from v2022dev0 to v2019)
Backport of !63 (merged)
The non-bootable state in LAVA is caused by automatic calculation of
the DTB name based on new naming schema by U-Boot.
The naming schema for Renesas boards has been changed by U-Boot and
kernel upstream. So need to provide the DTB name for old releases
explicitly or boot with run bootcmd_legacy_mmc0 command.
The former method is preferable since the latter is Apertis extension
for proper work of distro boot both with new and old kernels and not
compatible with other versions of U-Boot.