The default value for shutdown is less than 5% of battery life left.
Once the system hits this value (or the one specified by the user) the
system will shutdown.
The soc_shutdown value can be changed to 0 to disable this feature, just
realise that the system will lose power and unsafely shutdown.
Also added install and remove functionality.
Need to perform code cleanup, and make sure startup of the driver
contains everything it needs to properly verify the status of the
battery.
Could use code to get the voltage etc, however I don't see that code in
the argon40 python stuff so I'd have to investigate the I2C device.