Compress grub_prefix.

	Compress grub_prefix.

	* grub-core/boot/i386/pc/lnxboot.S: Use
	GRUB_KERNEL_I386_PC_MULTIBOOT_SIGNATURE.
	* grub-core/kern/i386/pc/startup.S: Move grub_prefix to compressed part.
	* include/grub/offsets.h: Rename GRUB_MACHINE_DATA_END to
	GRUB_MACHINE_PREFIX_END. All users updated.
	(GRUB_KERNEL_I386_PC_PREFIX): Set to GRUB_KERNEL_I386_PC_RAW_SIZE.
	(GRUB_KERNEL_I386_PC_PREFIX_END): Set to GRUB_KERNEL_I386_PC_PREFIX
	+ 0x40.
	(GRUB_KERNEL_I386_PC_RAW_SIZE): Decrease.
	* util/grub-mkimage.c (image_target_desc): Change data_end to
	prefix_end. All users updated.