grunt: Enable discharge on AC
Add support for setting the battery to discharge even if AC is
present. Used for factory testing.
BRANCH=none
TEST=ectool chargecontrol discharge, ectool battery
#define CONFIG_CHARGER
#define CONFIG_CHARGER_V2
#define CONFIG_CHARGE_MANAGER
+#define CONFIG_CHARGER_DISCHARGE_ON_AC
#define CONFIG_CHARGER_INPUT_CURRENT 128
#define CONFIG_CHARGER_ISL9238
#define CONFIG_CHARGER_MIN_BAT_PCT_FOR_POWER_ON 1
