lava: fix Renesas M3 boot with kernel 5.7

Test in LAVA with manually crafted image based on the kernel 5.7 shows it is booted well.

In commit 83772e1 and later commits DTB names in Linux kernel was renamed to align names with config options. Hence the old DTB name r8a7796-m3ulcb.dtb is 8a77960-ulcb.dtb for Renesas M3 boards used in LAVA.

Old U-Boot versions are using the old naming schema including version used in v2021dev3 atm.

This commit set proper name only for v2021dev3 branch allowing to boot images for Renesas boards in LAVA.

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