x86: generate EFI platform bootable images

Add EFI platform bootable images for x86 platforms. These images can
also boot from legacy BIOS platform.

EFI System Partition need to be fat12/fat16/fat32 (not need to load
filesystem drivers), so the first partition of EFI images are not ext4
filesystem any more.

GPT partition table has an alternate partition table, we did not
generate it. This may cause problems when use these images as qemu disk
(kernel can not find rootfs), we pad enough sectors will be ok.

Signed-off-by: 李国 <uxgood.org@gmail.com>
[part_magic_* refactoring, removed genisoimage checks]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/x86/base-files/lib/preinit/79_move_config

@@ -2,13 +2,14 @@
 # Copyright (C) 2012-2015 OpenWrt.org
 
 move_config() {
-	local partdev
+	local partdev parttype=ext4
 
 	. /lib/upgrade/common.sh
 
 	if export_bootdevice && export_partdevice partdev 1; then
 		mkdir -p /boot
-		mount -t ext4 -o rw,noatime "/dev/$partdev" /boot
+		part_magic_fat "/dev/$partdev" && parttype=vfat
+		mount -t $parttype -o rw,noatime "/dev/$partdev" /boot
 		if [ -f "/boot/$BACKUP_FILE" ]; then
 			mv -f "/boot/$BACKUP_FILE" /
 		fi

target/linux/x86/base-files/lib/preinit/81_upgrade_bootloader

@@ -6,6 +6,7 @@ upgrade_bootloader() {
 	. /lib/upgrade/common.sh
 
 	if [ ! -f /boot/grub/upgraded ] && export_bootdevice && export_partdevice diskdev 0; then
+		part_magic_efi "/dev/$diskdev" && return 0
 		echo "(hd0) /dev/$diskdev" > /tmp/device.map
 		/usr/sbin/grub-bios-setup \
 			-m "/tmp/device.map" \