change battery measurement, add battery monitor

src/hardware/pmu.h

@@ -3,6 +3,8 @@
 
     #define PMU_EVENT_AXP_INT       _BV(0)
 
+    #define PMU_BATTERY_CAP         380
+
     /*
      * @brief setup pmu: axp202
      * 
@@ -15,5 +17,14 @@
      * @param   ttgo    pointer to an TTGOClass
      */
     void pmu_loop( TTGOClass *ttgo );
+    /*
+     *
+     * @brief get the charge of battery in percent
+     * 
+     * @param   ttgo    pointer to an TTGOClass
+     * 
+     * @return  charge in percent
+     */
+    uint32_t pmu_get_byttery_percent( TTGOClass *ttgo );
 
 #endif // _PMU_H