chg: Reset wall current on start

Signed-off-by: Sean Cross <xobs@kosagi.com>
gg: Add precharge temperature reading/writing

Signed-off-by: Sean Cross <xobs@kosagi.com>
cmd-gg: Allow for manipulating precharge temperature

Signed-off-by: Sean Cross <xobs@kosagi.com>
chg: Support kickstarting gas gauge charging

Because the gas gauge will only enter charge mode when the current
detected is positive, we must make current positive when conditions
are such that we think we should charge.

Therefore, turn on the charger ever-so-slightly when:

    - The battery is discharging
    - The status flag indicates charging is allowed

Signed-off-by: Sean Cross <xobs@kosagi.com>