binman: Override CheckOptional in fit entry

Missing optional blobs was not reported for generated entries, e.g.
tee-os on rockchip targets. Implement a CheckOptional to fix this.

After this the following can be shown:

  Image 'simple-bin' is missing optional external blobs but is still functional: tee-os

  /binman/simple-bin/fit/images/@tee-SEQ/tee-os (tee-os):
     See the documentation for your board. You may need to build Open Portable
     Trusted Execution Environment (OP-TEE) and build with TEE=/path/to/tee.bin

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/binman/ftest.py

@@ -6358,6 +6358,13 @@ fdt         fdtmap                Extract the devicetree blob from the fdtmap
                          fdt_util.fdt32_to_cpu(node.props['entry'].value))
         self.assertEqual(U_BOOT_DATA, node.props['data'].bytes)
 
+        with test_util.capture_sys_output() as (stdout, stderr):
+            self.checkFitTee('264_tee_os_opt_fit.dts', '')
+        err = stderr.getvalue()
+        self.assertRegex(
+            err,
+            "Image '.*' is missing optional external blobs but is still functional: tee-os")
+
     def testFitTeeOsOptionalFitBad(self):
         """Test an image with a FIT with an optional OP-TEE binary"""
         with self.assertRaises(ValueError) as exc: