Battery Voltage

Readout/monitor battery voltage and turn off in case it gets too low. Also make the voltage available via API.

Partly implements #34.

epicardium/modules/buttons.c

 #include "epicardium.h"
+#include "modules/modules.h"
+#include "modules/log.h"
 
 #include "portexpander.h"
 #include "MAX77650-Arduino-Library.h"
@@ -15,12 +17,19 @@ uint8_t epic_buttons_read(uint8_t mask)
 {
 	uint8_t ret = 0;
 	if (portexpander_detected() && (mask & 0x7)) {
+		if (hwlock_acquire(HWLOCK_I2C, pdMS_TO_TICKS(100)) < 0) {
+			LOG_ERR("buttons", "Can't acquire I2C bus");
+			return 0;
+		}
+
 		/*
 		 * Not using PB_Get() here as that performs one I2C transcation
 		 * per button.
 		 */
 		uint8_t pin_status = ~portexpander_get();
 
+		hwlock_release(HWLOCK_I2C);
+
 		for (uint8_t m = 1; m < 0x8; m <<= 1) {
 			if (mask & m && pin_status & pin_mask[m]) {
 				ret |= m;