[geniso] Emit proper error message for incorrect location of ISOLINUX_BIN

If isolinux.bin is not installed in the expected location the error
message shown is slightly misleading.

Signed-off-by: Vibi Sreenivasan <vibi_sreenivasan@cms.com>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/util/geniso

 isolinux_bin=${ISOLINUX_BIN:-util/isolinux.bin}
 if [ ! -r $isolinux_bin ]
 then
-	echo $0: $isolinux_bin not found, please install, or set ISOLINUX_BIN in arch/i386/config correctly
+	echo $0: $isolinux_bin not found, please install, or set ISOLINUX_BIN in arch/i386/Makefile correctly
 	exit 1
 fi
 out=$1