binman: Use an exit code when blobs are missing

At present binman returns success when told to handle missing/faked blobs
or missing bintools. This is confusing since in fact the resulting image
cannot work.

Use exit code 103 to signal this problem, with a -W option to convert
it to a warning.

Rename the flag to --ignore-missing since it controls bintools also.

Add documentation about exit codes while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/control.py b/tools/binman/control.py
index bfe63a1..964c698 100644
--- a/tools/binman/control.py
+++ b/tools/binman/control.py
@@ -741,8 +741,15 @@
                 data = state.GetFdtForEtype('u-boot-dtb').GetContents()
                 elf.UpdateFile(*elf_params, data)
 
+            # This can only be True if -M is provided, since otherwise binman
+            # would have raised an error already
             if invalid:
-                tout.warning("\nSome images are invalid")
+                msg = '\nSome images are invalid'
+                if args.ignore_missing:
+                    tout.warning(msg)
+                else:
+                    tout.error(msg)
+                    return 103
 
             # Use this to debug the time take to pack the image
             #state.TimingShow()