lava: fix the name of DTB file for 18.12 boot

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.

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