spi: Added block-level SPI writes at the HAL level

Poking the block-level SPI writes through the HAL level gives more
freedom to targets to improve SPI transaction speed.

drivers/SPI.cpp

@@ -79,20 +79,11 @@ int SPI::write(int value) {
 }
 
 int SPI::write(const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length) {
-    int total = (tx_length > rx_length) ? tx_length : rx_length;
-
     lock();
     aquire();
-    for (int i = 0; i < total; i++) {
-        char out = (i < tx_length) ? tx_buffer[i] : 0xff;
-        char in = spi_master_write(&_spi, out);
-        if (i < rx_length) {
-            rx_buffer[i] = in;
-        }
-    }
+    int ret = spi_master_block_write(&_spi, tx_buffer, tx_length, rx_buffer, rx_length);
     unlock();
-
-    return total;
+    return ret;
 }
 
 void SPI::lock() {

hal/spi_api.h

@@ -116,6 +116,23 @@ void spi_frequency(spi_t *obj, int hz);
  */
 int  spi_master_write(spi_t *obj, int value);
 
+/** Write a block out in master mode and receive a value
+ *
+ *  The total number of bytes sent and recieved will be the maximum of
+ *  tx_length and rx_length. The bytes written will be padded with the
+ *  value 0xff.
+ *
+ * @param[in] obj       The SPI peripheral to use for sending
+ * @param[in] tx_buffer Pointer to the byte-array of data to write to the device
+ * @param[in] tx_length Number of bytes to write, may be zero
+ * @param[in] rx_buffer Pointer to the byte-array of data to read from the device
+ * @param[in] rx_length Number of bytes to read, may be zero
+ * @returns
+ *      The number of bytes written and read from the device. This is
+ *      maximum of tx_length and rx_length.
+ */
+int spi_master_block_write(spi_t *obj, const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length);
+
 /** Check if a value is available to read
  *
  * @param[in] obj The SPI peripheral to check