Convert CONFIG_CMD_SF_TEST to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SF_TEST

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 507e8a3..5ac8389 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -797,6 +797,17 @@
 	help
 	  SPI Flash support
 
+config CMD_SF_TEST
+	bool "sf test - Allow testing of SPI flash"
+	help
+	  Provides a way to test that SPI flash is working correctly. The
+	  test is destructive, in that an area of SPI flash must be provided
+	  for the test to use. Performance information is also provided,
+	  measuring the performance of reading, writing and erasing in
+	  Mbps (Million Bits Per Second). This value should approximately
+	  equal the SPI bus speed for a single-bit-wide SPI bus, assuming
+	  everything is working properly.
+
 config CMD_SPI
 	bool "sspi"
 	help