lib, vsprintf: introduce strict_strtoul

as checkpatch proposes to use strict_strtoul instead of
simple_strtoul, introduce it.

Ported this function from Linux 2.6.38 commit ID:
521cb40b0c44418a4fd36dc633f575813d59a43d

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
diff --git a/include/_exports.h b/include/_exports.h
index d89b65b..349a3c5 100644
--- a/include/_exports.h
+++ b/include/_exports.h
@@ -19,6 +19,7 @@
 EXPORT_FUNC(getenv)
 EXPORT_FUNC(setenv)
 EXPORT_FUNC(simple_strtoul)
+EXPORT_FUNC(strict_strtoul)
 EXPORT_FUNC(simple_strtol)
 EXPORT_FUNC(strcmp)
 EXPORT_FUNC(i2c_write)