Make SPI instance selectable (not needed, as it turns out)

src/sd_hw_config.c

@@ -38,9 +38,13 @@ socket, which SPI it is driven by, and how it is wired.
 // Hardware Configuration of SPI "objects"
 // Note: multiple SD cards can be driven by one SPI if they use different slave
 // selects.
+#ifndef SD_SPI_INST
+#define SD_SPI_INST (spi0)
+#endif
+
 static spi_t spis[] = {  // One for each SPI.
     {
-        .hw_inst = spi0,  // SPI component
+        .hw_inst = SD_SPI_INST,  // SPI component
         .miso_gpio = SD_RX, // GPIO number (not pin number)
         .mosi_gpio = SD_TX,
         .sck_gpio = SD_SCK,