[SPI] Make SPI_FLUSH() more versatile

Currently the SPI driver in core/ sets `SPI_FLUSH()` to be a single read
of the spi RX buffer. This is fine for many microcontrollers, but newer
platforms like the CC2538 have a multi-byte FIFO for the RX buffer. In
that case, a single read is not guaranteed to flush the RX. This commit
allows `SPI_FLUSH()` to be platform dependent if needed, but doesn't
change for platforms where it works.

core/dev/spi.h

@@ -82,10 +82,11 @@ void spi_init(void);
   } while(0)
 
 /* Flush the SPI read register */
+#ifndef SPI_FLUSH
 #define SPI_FLUSH() \
   do {              \
     SPI_RXBUF;      \
   } while(0);
-
+#endif
 
 #endif /* SPI_H_ */