Convert CONFIG_SYS_FLASH_EMPTY_INFO to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FLASH_EMPTY_INFO

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/mtd/Kconfig b/drivers/mtd/Kconfig
index 3d1f6e4..8c67193 100644
--- a/drivers/mtd/Kconfig
+++ b/drivers/mtd/Kconfig
@@ -106,6 +106,10 @@
 	help
 	  Use buffered writes to flash.
 
+config SYS_FLASH_EMPTY_INFO
+	bool "Enable displaying empty sectors in flash info"
+	depends on FLASH_CFI_DRIVER
+
 config FLASH_CFI_MTD
 	bool "Enable CFI MTD driver"
 	depends on FLASH_CFI_DRIVER