* include/grub/i386/pc/boot.h (GRUB_BOOT_MACHINE_PXE_DL): New macro.
* boot/i386/pc/pxeboot.S: Include `<grub/machine/boot.h>'.
(_start): Macroify `0x7F'.
* kern/i386/pc/init.c: Include `<grub/machine/boot.h>'.
(make_install_device): Use "(pxe)" as fallback prefix when booting
via PXE.