Convert CONFIG_SYS_USB_FAT_BOOT_PARTITION to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_USB_FAT_BOOT_PARTITION

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 42f2c95..3fd5644 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -1317,6 +1317,13 @@
 	  config options. This enables loading from USB using a configured
 	  device.
 
+config SYS_USB_FAT_BOOT_PARTITION
+	int "Partition on USB to use to load U-Boot from"
+	depends on SPL_USB_STORAGE
+	default 1
+	help
+	  Partition on the USB storage device to load U-Boot from
+
 config SPL_USB_GADGET
 	bool "Suppport USB Gadget drivers"
 	help