Export redesign

this is an atempt to make the export of functions typesafe.
I replaced the jumptable void ** by a struct (jt_funcs) with function pointers.
The EXPORT_FUNC macro now has 3 fixed parameters and one
variadic parameter
The first is the name of the exported function,
the rest of the parameters are used to format a functionpointer
in the jumptable,

the EXPORT_FUNC macros are expanded three times,
1. to declare the members of the struct
2. to initialize the structmember pointers
3. to call the functions in stubs.c

Signed-off-by: Martin Dorwig <dorwig@tetronik.com>
Acked-by: Simon Glass <sjg@chromium.org>

Signed-off-by: Simon Glass <sjg@chromium.org>
(resending to the list since my tweaks are not quite trivial)
diff --git a/common/cmd_load.c b/common/cmd_load.c
index f6e522c..d043e6d 100644
--- a/common/cmd_load.c
+++ b/common/cmd_load.c
@@ -222,7 +222,7 @@
 		}
 
 	    /* Check for the console hangup (if any different from serial) */
-	    if (gd->jt[XF_getc] != getc) {
+	    if (gd->jt->getc != getc) {
 		if (ctrlc()) {
 		    return (-1);
 		}