spi: tools: move to tools buildsystem

There is a nice buildsystem dedicated for userspace tools in Linux kernel tree.
Switch spi target to be built by it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/tools/spi/Build b/tools/spi/Build
new file mode 100644
index 0000000..8e84660
--- /dev/null
+++ b/tools/spi/Build
@@ -0,0 +1,2 @@
+spidev_test-y += spidev_test.o
+spidev_fdx-y += spidev_fdx.o