refactor: Move drivers into properly module.

* Align our driver module layout to properly match Zephyr conventions,
  allowing proper CMake setup to amend the library for each type of driver.

app/module/drivers/sensor/battery/battery_common.c

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <errno.h>
+#include <zephyr/drivers/sensor.h>
+
+#include "battery_common.h"
+
+int battery_channel_get(const struct battery_value *value, enum sensor_channel chan,
+                        struct sensor_value *val_out) {
+    switch (chan) {
+    case SENSOR_CHAN_GAUGE_VOLTAGE:
+        val_out->val1 = value->millivolts / 1000;
+        val_out->val2 = (value->millivolts % 1000) * 1000U;
+        break;
+
+    case SENSOR_CHAN_GAUGE_STATE_OF_CHARGE:
+        val_out->val1 = value->state_of_charge;
+        val_out->val2 = 0;
+        break;
+
+    default:
+        return -ENOTSUP;
+    }
+
+    return 0;
+}
+
+uint8_t lithium_ion_mv_to_pct(int16_t bat_mv) {
+    // Simple linear approximation of a battery based off adafruit's discharge graph:
+    // https://learn.adafruit.com/li-ion-and-lipoly-batteries/voltages
+
+    if (bat_mv >= 4200) {
+        return 100;
+    } else if (bat_mv <= 3450) {
+        return 0;
+    }
+
+    return bat_mv * 2 / 15 - 459;
+}