Repurposed sppp_power as battery driver, updated config.
We guess battery charge by measuring current voltage and comparing to a discharge graph.
/sys/class/power_supply/sppp_power show battery data and DCIN changes create device events.
Enable SPPP core to handle multiple client tx.

This is done with a kfifo of 'active' messages that is regularly cleared
and sent to the STM using a kfifo. Introduces a new SPPP API.
Merge pull request #5 from genesi/slimbook-sppp-core_ESS-197

SPPP Core - Enable multiple clients (ESS-197)