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>
diff --git a/cmd/cedit.c b/cmd/cedit.c
index 0cae304..5f0e844 100644
--- a/cmd/cedit.c
+++ b/cmd/cedit.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <cedit.h>
 #include <command.h>
 #include <expo.h>
 #include <fs.h>