kconfig: Warn if there is more than one help text

Avoids mistakes like in the following real-world example, where only the
final help string ("Say Y...") was used. This particular example was
fixed in commit 561b29e4 ("media: fix media Kconfig help syntax
issues").

  config DVB_NETUP_UNIDVB
  	...
	select DVB_CXD2841ER if MEDIA_SUBDRV_AUTOSELECT
  	---help---
  	  Support for NetUP PCI express Universal DVB card.
       help
  	Say Y when you want to support NetUP Dual Universal DVB card
        ...

This now prints the following warning:

  drivers/media/pci/netup_unidvb:13: warning: 'DVB_NETUP_UNIDVB' defined with more than one help text -- only the last one will be used

Also free() any extra help strings.
Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/zconf.y

@@ -431,6 +431,11 @@ help_start: T_HELP T_EOL
 
 help: help_start T_HELPTEXT
 {
+	if (current_entry->help) {
+		free(current_entry->help);
+		zconfprint("warning: '%s' defined with more than one help text -- only the last one will be used",
+			   current_entry->sym->name ?: "<choice>");
+	}
 	current_entry->help = $2;
 };