When opkg is used as package manager and systemd is used as init system there is a problem with restarting systemd service on upgrading.
opkg doesn't call prerm script which should stop old service but calls postinst script that starts service. 
Therefore new service won't start since old one wasn't stopped.

Steps to reproduce:
1) Build a systemd based image with opkg package manager.
2) Start image.
3) Build any package that has systemd service (e.g. connman).
4) Copy two packages that was builded (e.g. "connman" and "connman-systemd") from deploy/ipk/<arch> directory into image.
5) Make "opkg install <path to copied packages>/connman*" on image
6) Remove these packages (they do not needed anymore)
7) Increase package revision of connman.
8) Do step 3 - 5.
9) Make "opkg install <path to copied packages>/connman*" on image

Expected behavior:
In journalctl you have to see a message about stopping and starting connman service.

Actual behavior:
In journalctl you have to see an *error* message about starting connman service (service wasn't stopped).