Convert CONFIG_SYS_MMCSD_FS_BOOT_PARTITION to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MMCSD_FS_BOOT_PARTITION

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index dcc1a95..72c7165 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -680,6 +680,13 @@
 	  this option to build the drivers in drivers/mmc as part of an SPL
 	  build.
 
+config SYS_MMCSD_FS_BOOT_PARTITION
+	int "MMC Boot Partition"
+	default 1
+	help
+	  Partition on the MMC to load U-Boot from when the MMC is being
+          used in fs mode
+
 config SPL_MMC_TINY
 	bool "Tiny MMC framework in SPL"
 	depends on SPL_MMC_SUPPORT