x86/build: Specify -input-charset=utf-8 for mkisofs

It avoids the following warning triggered by newer versions of mkisofs:

    -input-charset not specified, using utf-8 (detected in locale settings)
Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yamada.masahiro@socionext.com
Link: http://lkml.kernel.org/r/1509939179-7556-4-git-send-email-changbin.du@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/genimage.sh

@@ -103,8 +103,9 @@ genisoimage() {
 	if [ -f "$FDINITRD" ] ; then
 		cp "$FDINITRD" $tmp_dir/initrd.img
 	fi
-	mkisofs -J -r -o $FIMAGE -b isolinux.bin -c boot.cat \
-		-no-emul-boot -boot-load-size 4 -boot-info-table $tmp_dir
+	mkisofs -J -r -input-charset=utf-8 -o $FIMAGE -b isolinux.bin \
+		-c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table \
+		$tmp_dir
 	isohybrid $FIMAGE 2>/dev/null || true
 	rm -rf $tmp_dir
 }