riscv: do not set default fdt for VisionFive 2

riscv: do not set default fdt for VisionFive 2

Currently in set_fdtfile() we set the value of environment variable fdtfile
unconditionally. The implies that a value in the environment will be
ignored.

With the patch environment variable fdtfile will only be set if it does not
yet exist. This requires that CONFIG_DEFAULT_FDT_FILE is not set.

Now the user can either set and save fdtfile interactively or in the U-Boot
configuration to overrule the device-tree name chosen based on the
hardware in set_fdtfile().

Reported-by: E Shattow <lucent@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>