bitbake/ui/knotty: Ensure previous failures aren't masked by a final successful command

When running bitbake in -k mode, the last command might succeed but we
still need to preserve any previous failure codes.

(Bitbake rev: 3ad3a53545b37ee8b26b22f4a0e00b19615b449a)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/ui/knotty.py

@@ -188,7 +188,8 @@ def main(server, eventHandler):
                 logger.error("Command execution failed: %s", event.error)
                 break
             if isinstance(event, bb.command.CommandExit):
-                return_value = event.exitcode
+                if not return_value:
+                    return_value = event.exitcode
                 continue
             if isinstance(event, bb.cooker.CookerExit):
                 break