Use the same reboot approach on i386 coreboot and qemu as we do on
BIOS.
* conf/i386-coreboot.rmk (kernel_img_HEADERS): Add `cpu/reboot.h'.
(reboot_mod_SOURCES): Remove `kern/i386/reboot.c'.
* kern/i386/reboot.c: Remove.
* include/grub/i386/reboot.h (grub_reboot): Export function.
* kern/i386/pc/startup.S (grub_reboot): Move from here ...
* kern/i386/realmode.S (grub_reboot): ... to here. Jump to
0xf000:0xfff0 instead of 0xffff:0x0000.
[!GRUB_MACHINE_PCBIOS] (prot_to_real): Do not restore interrupts.
* kern/i386/qemu/startup.S: Include `"../realmode.S"'.