[cob_bms_driver] charging flag cannot be used for docking status

[floweisshardt]

see discussion in https://github.com/ipa320/msh/issues/595#issuecomment-277238899.

this was caused by the charging flag from the bms switching to false as soon as the battery is full because there is no more current going into the battery.

Proper solution is to use a combination of the charging and battery_full flags and generate a docked status out of it. the PowerState message should be enhanced to have this additional flag.

@fmessmer FYI

[fmessmer]

related workaround https://github.com/ipa320/cob_driver/pull/307 with discussion