tsc: Use alternative delay sources whenever appropriate.

PIT isn't available on some of new hardware including Hyper-V. So
use pmtimer for calibration. Moreover pmtimer calibration is faster, so
use it on coreboor where booting time is important.

Based on patch by Michael Chang.

grub-core/kern/acpi.c

@@ -0,0 +1,34 @@
+/* 
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2012  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/types.h>
+#include <grub/time.h>
+#include <grub/misc.h>
+#include <grub/acpi.h>
+
+/* Simple checksum by summing all bytes. Used by ACPI and SMBIOS. */
+grub_uint8_t
+grub_byte_checksum (void *base, grub_size_t size)
+{
+  grub_uint8_t *ptr;
+  grub_uint8_t ret = 0;
+  for (ptr = (grub_uint8_t *) base; ptr < ((grub_uint8_t *) base) + size;
+       ptr++)
+    ret += *ptr;
+  return ret;
+}