Convert CONFIG_CMD_FDC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_FDC

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index cc959f6..99082c1 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -604,6 +604,11 @@
 	help
 	  GPIO support.
 
+config CMD_FDC
+	bool "fdcboot - Boot from floppy device"
+	help
+	  The 'fdtboot' command allows booting an image from a floppy disk.
+
 endmenu