kconfig: attach help text to menus

Roman Zippel wrote:
> A simple example would be
> help texts, right now they are per symbol, but they should really be per
> menu, so archs can provide different help texts for something.

This patch does this and at the same time introduce a few API
funtions used to access the help text.

The relevant api functions are introduced in the various frontends.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>

scripts/kconfig/expr.h

@@ -71,7 +71,6 @@ enum {
 struct symbol {
 	struct symbol *next;
 	char *name;
-	char *help;
 	enum symbol_type type;
 	struct symbol_value curr;
 	struct symbol_value def[4];
@@ -139,7 +138,7 @@ struct menu {
 	struct property *prompt;
 	struct expr *dep;
 	unsigned int flags;
-	//char *help;
+	char *help;
 	struct file *file;
 	int lineno;
 	void *data;