expo: Split out cedit into its own header

Before adding more functions to this interface, create a new header for
the configuration editor.

Fix up the expo header guard while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/cedit.c

@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <cedit.h>
 #include <command.h>
 #include <expo.h>
 #include <fs.h>