battery: Implement charging thresholds

Battery Charging Start Threshold:
- Valid values are [0, 100]
- A value of 0 disables the feature, leaving the charger enabled on AC

Battery Charging End Threshold:
- Valid values are [0,100]
- A value of 100 disabled the feature, leaving the charger enabled on AC

The stop threshold must be configured for the start threshold to work.

src/board/system76/common/power.c

@@ -357,7 +357,7 @@ void power_event(void) {
             battery_charger_disable();
         } else {
             DEBUG("plugged in\n");
-            battery_charger_enable();
+            battery_charger_configure();
         }
         battery_debug();
 
@@ -377,6 +377,11 @@ void power_event(void) {
 
     gpio_set(&AC_PRESENT, !ac_new);
 
+    // Configure charger based on charging thresholds when plugged in
+    if (!ac_new) {
+        battery_charger_configure();
+    }
+
     // Read power switch state
     static bool ps_last = true;
     bool ps_new = gpio_get(&PWR_SW_N);