unlink old database file before creating a new one

udev_db_add_device() can be called when the corressponding database
entry already exists - it should overwrite the old entry in this case.
However, if the old entry was a symlink, fopen(filename, "w") will not
overwrite it properly - it will keep the symlink and create a file
named after the symlink target.  Calling unlink(filename) before
trying to create the database file fixes the problem.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>

udev_db.c

@@ -93,6 +93,7 @@ int udev_db_add_device(struct udevice *udev)
 		struct name_entry *name_loop;
 		FILE *f;
 
+		unlink(filename);
 		f = fopen(filename, "w");
 		if (f == NULL) {
 			err("unable to create db file '%s': %s", filename, strerror(errno));