what to do to prevent my bbb living his own live?

I haven't looked into all the things the BBB does during boot, but as a general trick to reduce the boot time on Linux you can check how long the boot takes with systemd-analyze. When I do this on the BBB I found that it takes 39 seconds to finish booting.

$ systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.

graphical.target @39.525s
└─multi-user.target @39.519s
  └─getty.target @39.187s
    └─serial-getty@ttyGS0.service @39.154s
      └─dev-ttyGS0.device @39.137s

So you can see that starting ttyGS0 took 39 seconds, there's also a web server running on the virtual USB network interface. Use systemctl status to list all services on your system.

When I disable the bonescript-autorun service I reach graphical.target in 27 seconds, so that's 12 seconds shaved off the boot time (don't worry about it being called graphical target, if you're using the IoT image there's no GUI)

$ sudo systemctl disable bonescript-autorun.service

When I also disable the web server nginx it doesn't shave off any time on boot. This is because services are starting even after the login prompt is visible.

$ sudo systemctl disable nginx.service

So I don't think DHCP being accessible would be related to the "long" boot time. Then what's taking up so much time? Well, we can check everything that has to be started before multi-user.target is reached.

$ systemctl list-dependencies multi-user.target

It will show a long list, some of those things are not enabled or running so you'll have to do some research into what you can disable. I choose two I didn't think was needed:

$ sudo systemctl disable bb-bbai-tether.service
$ sudo systemctl disable serial-getty@ttyGS0.service # this is a generated symlink, so it need to be masked to be disabled
$ sudo systemctl mask serial-getty@ttyGS0.service # this will create a symlink to /dev/null, disabling it forever

End result of just disabling services?

$ systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.

graphical.target @15.297s
└─multi-user.target @15.292s
  └─getty.target @15.070s
    └─serial-getty@ttyS0.service @15.054s
      └─dev-ttyS0.device @15.026s

Enjoy your extra 15 seconds of free time. I'll just end with this:

$ systemd-analyze 
Startup finished in 8.227s (kernel) + 15.522s (userspace) = 23.749s

Unpacking the kernel takes a few seconds and disabling services might break things. I'm not taking any responsibility if disabling services breaks your system.

It can possible in pinnless system in motherboard

Who is that guy

      ajcc   ?

You are my hero!

Very detailed and very helpful.

Your instructions gave me some hints where to look further and finally I've got my network running as I want and it takes 18s to come up.

very very nice.