rak3172: auto-detect crystal vs TCXO

Pass a TCXO voltage to begin() so RadioLib probes for one and falls back
to the crystal if it does not start. One build then serves the crystal
RAK3172 and the TCXO -T alike, where a hard-coded 0 fails init with -707.

variants/rak3x72/platformio.ini

@@ -10,7 +10,7 @@ build_flags = ${stm32_base.build_flags}
   -D RX_BOOSTED_GAIN=true
   -D STM32WL_ENABLE_SMPS ; run the radio from the SMPS/DC-DC regulator (needs the VLXSMPS coil)
   -D STM32WL_SLEEP_STOP0 ; sleep in Stop0 so the SMPS stays active during continuous RX
-;  -D STM32WL_TCXO_VOLTAGE=1.6 ; defaults to 0 if undef
+;  -D STM32WL_TCXO_VOLTAGE=1.8 ; override the auto-detected default from target.cpp
 ;  -D LORA_TX_POWER=14 ; Defaults to 22 for HP, 14 is for LP version
   -I variants/rak3x72
 build_src_filter = ${stm32_base.build_src_filter}

variants/rak3x72/target.cpp

@@ -24,9 +24,11 @@ SensorManager sensors;
   #define LORA_CR      5
 #endif
 
+// Auto-detect the oscillator. begin() is given a TCXO voltage; RadioLib falls
+// back to the crystal if it fails to start, so one build runs on both the
+// crystal RAK3172 and the TCXO -T. A hard-coded 0 leaves the -T failing (-707).
 #ifndef STM32WL_TCXO_VOLTAGE
-  // TCXO set to 0 for RAK3172
-  #define STM32WL_TCXO_VOLTAGE 0
+  #define STM32WL_TCXO_VOLTAGE 1.8
 #endif
 
 #ifndef LORA_TX_POWER