* include/grub/i386/efi/machine.h: New file.
* include/grub/i386/linuxbios/machine.h: Likewise.
* include/grub/i386/pc/machine.h: Likewise.
* include/grub/powerpc/ieee1275/machine.h: Likewise.
* include/grub/sparc64/ieee1275/machine.h: Likewise.
* term/i386/pc/serial.c: Include <grub/machine/machine.h>.
(serial_hw_io_addr): New variable.
(serial_hw_get_port): Obtain port address from `serial_hw_io_addr'
instead of `(unsigned short *) 0x400'.
