DuckRadio.cpp compile error when using SX126X

[amirna2]

Describe the bug When building for a board with a Semtech SX1262 the build fails, unless SPI pins are defined in the board definition. SPI should always be optional and it is not needed in most cases, because the boards we support all have a SoC that integrates the LoRa chip with the CPU and in which case the SPI communication is handled by RadioLib internally.

The failing code is

#if defined(CDPCFG_PIN_LORA_SCLK) & defined(CDPCFG_RADIO_SX126X)
#include <SPI.h>
SPIClass spi;
SPISettings spiSettings;
CDPCFG_LORA_CLASS lora =
        new Module(CDPCFG_PIN_LORA_CS, CDPCFG_PIN_LORA_DIO1, CDPCFG_PIN_LORA_RST,
                   CDPCFG_PIN_LORA_BUSY, spi);
#elif defined(CDPCFG_RADIO_SX126X)
CDPCFG_LORA_CLASS lora =
        new Module(CDPCFG_PIN_LORA_CS, CDPCFG_PIN_LORA_DIO1, CDPCFG_PIN_LORA_RST,
                   CDPCFG_PIN_LORA_BUSY, spi);
#else
CDPCFG_LORA_CLASS lora = new Module(CDPCFG_PIN_LORA_CS, CDPCFG_PIN_LORA_DIO0,
                  CDPCFG_PIN_LORA_RST, CDPCFG_PIN_LORA_DIO1);
#endif

Notice that both #if and #elif blocks are identical, however the #elif block fails to compile if CDPCFG_RADIO_SX126X is defined. Which is the most common case.

To reproduce

1. Choose the Heltec CubeCell for example (it doesn't define the SPI pins)
2. The compilation will fail at this line beccause spi is undefined (it's defined in the #if block not in the #elif

new Module(CDPCFG_PIN_LORA_CS, CDPCFG_PIN_LORA_DIO1, CDPCFG_PIN_LORA_RST,
                   CDPCFG_PIN_LORA_BUSY, spi);

Expected behavior When building for an SX1262 board and we don't defined SPI pins, DuckRadio should build.

Additional context The fix for this is to not use SPI in #elif defined(CDPCFG_RADIO_SX126X)

new Module(CDPCFG_PIN_LORA_CS, CDPCFG_PIN_LORA_DIO1, CDPCFG_PIN_LORA_RST,
                   CDPCFG_PIN_LORA_BUSY);

Additionally, instead of checking for the SCLK define here, it's better to use a more explicit definition e.g USE_SPI which should be defined in the board definition header file.

#if defined(CDPCFG_PIN_LORA_SCLK) & defined(CDPCFG_RADIO_SX126X)