move CLI prototypes to cli.h and add comments

Move the CLI prototypes from common.h to cli.h as part of an effort to
reduce the size of common.h.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/hymod/input.c b/board/hymod/input.c
index 184902c..23d3f19 100644
--- a/board/hymod/input.c
+++ b/board/hymod/input.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <cli.h>
 
 int
 hymod_get_serno (const char *prompt)