xen: Add basic hooks for PVH in current code

Add the hooks to current code needed for Xen PVH. They will be filled
with code later when the related functionality is being added.

loader/i386/linux.c needs to include machine/kernel.h now as it needs
to get GRUB_KERNEL_USE_RSDP_ADDR from there. This in turn requires to
add an empty kernel.h header for some i386 platforms (efi, coreboot,
ieee1275, xen) and for x86_64 efi.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Hans van Kranenburg <hans@knorrie.org>

grub-core/kern/i386/xen/startup_pvh.S

@@ -0,0 +1,29 @@
+/* startup.S - bootstrap GRUB itself */
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2018  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 <config.h>
+#include <grub/symbol.h>
+
+	.file	"startup_pvh.S"
+	.text
+
+/* Saved pointer to start info structure. */
+	.globl	pvh_start_info
+pvh_start_info:
+	.long	0