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>
diff --git a/board/starfive/visionfive2/starfive_visionfive2.c b/board/starfive/visionfive2/starfive_visionfive2.c
index 78e118d..7643917 100644
--- a/board/starfive/visionfive2/starfive_visionfive2.c
+++ b/board/starfive/visionfive2/starfive_visionfive2.c
@@ -49,6 +49,10 @@
 	u8 version;
 	const char *fdtfile;
 
+	fdtfile = env_get("fdtfile");
+	if (fdtfile)
+		return;
+
 	version = get_pcb_revision_from_eeprom();
 	switch (version) {
 	case 'a':