Redesign sleep() functions

- unified sleeping function _sleep(), including smartSleep
- prevent sleeping if transport is not ready
- add debug log messages

core/MyHwSAMD.cpp

@@ -117,7 +117,7 @@ void hwReboot() {
 int8_t hwSleep(unsigned long ms) {
   // TODO: Not supported!
   (void)ms;
-  return -2;
+  return MY_SLEEP_NOT_POSSIBLE;
 }
 
 int8_t hwSleep(uint8_t interrupt, uint8_t mode, unsigned long ms) {
@@ -125,7 +125,7 @@ int8_t hwSleep(uint8_t interrupt, uint8_t mode, unsigned long ms) {
   (void)interrupt;
   (void)mode;
   (void)ms;
-  return -2;
+  return MY_SLEEP_NOT_POSSIBLE;
 }
 
 int8_t hwSleep(uint8_t interrupt1, uint8_t mode1, uint8_t interrupt2, uint8_t mode2, unsigned long ms) {
@@ -135,7 +135,7 @@ int8_t hwSleep(uint8_t interrupt1, uint8_t mode1, uint8_t interrupt2, uint8_t mo
   (void)interrupt2;
   (void)mode2;
   (void)ms;
-  return -2;
+  return MY_SLEEP_NOT_POSSIBLE;
 }
 
 uint16_t hwCPUVoltage() {