Open Chrome from terminal with developer console open
The flag you're looking for is --auto-open-devtools-for-tabs. Please note, that you should quit Chrome before this setting will take effect.
This has worked at least since Chrome 55.0.2883.87 m (the latest version as of initial post)
A Google employee maintains this list of automatically updated Chromium flags. There doesn't seem to be a flag for opening Chrome with developer console open.
What exactly are you trying to achieve by that? If you are trying to run some kind of automated tests, you should try out PhantomJS.