test: py: add mkimage test for undefined image references in FIT configs

Add a test case to verify that mkimage correctly rejects a FIT source
that references a non-existent image from a configuration node.

This test introduces a minimal ITS that defines a valid kernel image
but references a missing "fdt" image under the /configurations section.
The test asserts that mkimage fails with a clear error message, as
introduced in the new validation logic.

This helps ensure the validation logic behaves correctly and prevents
regressions in future FIT enhancements.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>

test/py/tests/test_fit_mkimage_validate.py

@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: GPL-2.0+
+# Copyright (c) 2025
+#
+# Test that mkimage validates image references in configurations
+
+import os
+import subprocess
+import pytest
+import fit_util
+
+@pytest.mark.boardspec('sandbox')
+@pytest.mark.requiredtool('dtc')
+def test_fit_invalid_image_reference(ubman):
+    """Test that mkimage fails when configuration references a missing image"""
+
+    its_fname = fit_util.make_fname(ubman, "invalid.its")
+    itb_fname = fit_util.make_fname(ubman, "invalid.itb")
+    kernel = fit_util.make_kernel(ubman, 'kernel.bin', 'kernel')
+
+    # Write ITS with an invalid reference to a nonexistent image
+    its_text = '''
+/dts-v1/;
+
+/ {
+    images {
+        kernel@1 {
+            description = "Test Kernel";
+            data = /incbin/("kernel.bin");
+            type = "kernel";
+            arch = "sandbox";
+            os = "linux";
+            compression = "none";
+            load = <0x40000>;
+            entry = <0x40000>;
+        };
+    };
+
+    configurations {
+        default = "conf@1";
+        conf@1 {
+            kernel = "kernel@1";
+            fdt = "notexist";
+        };
+    };
+};
+'''
+
+    with open(its_fname, 'w') as f:
+        f.write(its_text)
+
+    mkimage = os.path.join(ubman.config.build_dir, 'tools/mkimage')
+    cmd = [mkimage, '-f', its_fname, itb_fname]
+
+    result = subprocess.run(cmd, capture_output=True, text=True)
+
+    assert result.returncode != 0, "mkimage should fail due to missing image reference"
+    assert "references undefined image 'notexist'" in result.stderr
+