Make it possible to define TCXO and XTAL radio modules within one variant (#4492)

* Update main.cpp

Add in TCXO_OPTIONAL variable for tcxoVoltage and a double-check for working in both modes.

* Update SX126xInterface.cpp

Make a change to the tcxoVoltage setting so that TCXO_OPTIONAL works if defined.

* Update variant.h

Added define for TCXO_OPTIONAL and the tcxoVoltage variable.

Added detail on the compatible boards.

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/SX126xInterface.cpp

@@ -40,7 +40,7 @@ template <typename T> bool SX126xInterface<T>::init()
         0; // "TCXO reference voltage to be set on DIO3. Defaults to 1.6 V, set to 0 to skip." per
            // https://github.com/jgromes/RadioLib/blob/690a050ebb46e6097c5d00c371e961c1caa3b52e/src/modules/SX126x/SX126x.h#L471C26-L471C104
     // (DIO3 is free to be used as an IRQ)
-#else
+#elif !defined(TCXO_OPTIONAL)
     float tcxoVoltage = SX126X_DIO3_TCXO_VOLTAGE;
     // (DIO3 is not free to be used as an IRQ)
 #endif
@@ -345,4 +345,4 @@ template <typename T> bool SX126xInterface<T>::sleep()
 #endif
 
     return true;
-}
+}