samples: hello_world_user: unify board identifier output

This patch unifies the board target identifier output to match that in
the `hello_world/src/main.c` file.

Signed-off-by: Patryk Koscik <pkoscik@antmicro.com>

samples/userspace/hello_world_user/src/main.c

@@ -19,7 +19,7 @@ static void user_function(void *p1, void *p2, void *p3)
 {
 	printf("Hello World from %s (%s)\n",
 	       k_is_user_context() ? "UserSpace!" : "privileged mode.",
-	       CONFIG_BOARD);
+	       CONFIG_BOARD_TARGET);
 	__ASSERT(k_is_user_context(), "User mode execution was expected");
 }