kconfig: Warn if help text is blank

Blank help texts are probably either a typo, a Kconfig misunderstanding,
or some kind of half-committing to adding a help text (in which case a
TODO comment would be clearer, if the help text really can't be added
right away).

Best to flag them, IMO.

Example warning:

	drivers/mmc/host/Kconfig:877: warning: 'MMC_TOSHIBA_PCI' defined with blank help text
Signed-off-by: NUlf Magnusson <ulfalizer@gmail.com>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/zconf.y

@@ -436,6 +436,12 @@ help: help_start T_HELPTEXT
 		zconfprint("warning: '%s' defined with more than one help text -- only the last one will be used",
 			   current_entry->sym->name ?: "<choice>");
 	}
+
+	/* Is the help text empty or all whitespace? */
+	if ($2[strspn($2, " \f\n\r\t\v")] == '\0')
+		zconfprint("warning: '%s' defined with blank help text",
+			   current_entry->sym->name ?: "<choice>");
+
 	current_entry->help = $2;
 };