U-Boot with distroboot has:
efi_dtb_prefixes=/ /dtb/ /dtb/current/
So we should install the device-trees into dtb/ and not dtbs/ on the EFI
system partition.
Fixes: 365435ad2dbe ("riscv: copy device trees to the ESP")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>