lorawan cleanup and optimizations:

- Remove all default loraconf default initialisation in lorawan examples (Region Settings takes care of this now)
- Remove retries while receiving JoinAccept and increase LoraRX to 10s (no need to wait more than 10s)
- Add constants for RegionSetting default frequencies
- Standardize all lora default configurations in lora examples
- Add new AT+LW=NET,(ON|OFF) command in atcmd example

sx126x/sx126x.go

@@ -576,6 +576,12 @@ func (d *Device) SetTxPower(txpow int8) {
 	d.loraConf.LoraTxPowerDBm = txpow
 }
 
+// SetHeaderType sets implicit or explicit header mode
+// NB: Change will be applied at next RX / TX
+func (d *Device) SetHeaderType(headerType uint8) {
+	d.loraConf.HeaderType = headerType
+}
+
 //
 // Lora functions
 //