switch-branch and opkg: opkg postinst script fails to verify/fix branch setting because of locked opkg

When opkg is updated it overrides /etc/opkg/distfeeds.conf. postinst script contains for that reason call to switch-branch --verify. Problem is that this call fails when called as part of updater process because opkg is unable to lock its lock.

We should drop all usage of opkg in switch-branch with exception of installations because of that.