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/test/264_tee_os_opt_fit.dts

@@ -25,6 +25,7 @@
 					fit,data;
 
 					tee-os {
+						optional;
 					};
 				};
 			};