Introduce CONFIG_CHECK_SIGNATURE

Introduce CONFIG_CHECK_SIGNATURE to control inclusion of check_signature()
and avoid problems on platforms that don't have readb().

Let the few legacy (ISA || PCI || X86) drivers that need check_signature()
select CONFIG_CHECK_SIGNATURE.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index ef32e97..4245b7f 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -68,6 +68,7 @@
 config BLK_DEV_XD
 	tristate "XT hard disk support"
 	depends on ISA && ISA_DMA_API
+	select CHECK_SIGNATURE
 	help
 	  Very old 8 bit hard disk controllers used in the IBM XT computer
 	  will be supported if you say Y here.