* boot/i386/pc/boot.S: Remove root_drive. Assert offset of
boot_drive_check by using GRUB_BOOT_MACHINE_DRIVE_CHECK. Don't
save %dx, we only need %dl and we never change it.
* boot/i386/pc/cdboot.S: Don't set the root drive.
* boot/i386/pc/pxeboot.S: Likewise.
* include/grub/i386/pc/boot.h: Remove
GRUB_BOOT_MACHINE_ROOT_DRIVE, adjust
GRUB_BOOT_MACHINE_DRIVE_CHECK.
* include/grub/i386/pc/kernel.h: Remove grub_root_drive.
* kern/i386/pc/init.c (make_install_device): Remove references
to grub_root_drive.
* kern/i386/pc/startup.S: Likewise.
* util/i386/pc/grub-setup.c (setup): Don't set root_drive.
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