Add sleep forever by sleep(0);

For battery powered sensors which wake up by the RST Pin its useful
to sleep the cpu forever. When using this command:
sleep(INTERRUPT_NOT_DEFINED, MODE_NOT_DEFINED, 0, false);
the code will be stripped down to hwSleep(0);.
But this will not sleep the cpu forever.

hal/architecture/AVR/MyHwAVR.cpp

@@ -141,8 +141,14 @@ void hwInternalSleep(uint32_t ms)
 
 int8_t hwSleep(uint32_t ms)
 {
-	hwInternalSleep(ms);
-	return MY_WAKE_UP_BY_TIMER;
+	if (ms > 0u) {
+		// sleep for defined time
+		hwInternalSleep(ms);
+		return MY_WAKE_UP_BY_TIMER;
+	} else {
+		// sleep until ext interrupt triggered
+		hwPowerDown(WDTO_SLEEP_FOREVER);
+	}
 }
 
 int8_t hwSleep(const uint8_t interrupt, const uint8_t mode, uint32_t ms)