common: Add CMD_SF Kconfig entry

Added Kconfig entry for CMD_SF.

Signed-off-by: Jagan Teki <jteki@openedev.com>
diff --git a/common/Kconfig b/common/Kconfig
index 2976cd7..40cd69e 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -311,6 +311,11 @@
 	help
 	  NAND support.
 
+config CMD_SF
+	bool "sf"
+	help
+	  SPI Flash support
+
 config CMD_SPI
 	bool "sspi"
 	help