Writing documentation related to SPI slave.

Fix build issues

Documentation/applications/examples/spislv_test/index.rst

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+``spislv`` SPI slave example
+============================
+
+A simple example for the device functioning as an SPI slave. 
+This example can be used to validate communication with another device 
+operating as an SPI master. If the spitool is used on the other device,
+and the following command is sent:
+
+``spi exch -x 4 deadbeef``
+
+The expected response in device running spislv_test app is: 
+
+.. code-block:: bash
+
+  Slave: 4 Bytes reads
+  Value in hex form from /dev/spislv2: de ad be ef
+  Slave: Writing value back to /dev/spislv2
+
+
+This test requires the device to be configured in SPI slave mode.(your
+specific spi slave hardware settings might require additional settings).
+
+Specific configuration options for this example include:
+Configs inside <> are the options that you need to find, It are arch de-
+pendent and might be different for your board.
+
+- ``<Enable SPI Peripheral Support>`` – Enables SPI peripheral support
+  in System Type->Peripheral Support.
+
+- ``<Configure SPI>`` – Configures SPI peripheral in System Type->SPI
+  configuration. Set here the right pins to be used for SPI communication.
+
+- ``CONFIG_SPI`` – In the SPI driver support, enable SPI Slave support.
+
+- ``CONFIG_SPI_SLAVE`` – Enables SPI Slave support.
+
+- ``SPI_SLAVE_DRIVER`` – Enables SPI Slave character driver.
+
+- ``<Configure SPI>`` – Return to this option and enable SPI Slave mode.
+
+
+