Use newly-added Multiboot support in coreboot.
* conf/i386-coreboot.rmk (kernel_elf_SOURCES): Replace
`kern/i386/coreboot/mmap.c' with `kern/i386/multiboot_mmap.c'.
* kern/i386/coreboot/startup.S: Enable Multiboot header, fix its
alignment, set `MULTIBOOT_MEMORY_INFO' flag.
(codestart): Store the MBI in `startup_multiboot_info' when we're
being loaded using Multiboot.
* kern/i386/coreboot/init.c (grub_machine_init): Move
grub_at_keyboard_init() call to beginning of function (useful for
debugging). Call grub_machine_mmap_init() before attempting to use
grub_machine_mmap_iterate().
(grub_lower_mem, grub_upper_mem): Move from here ...
* kern/i386/multiboot_mmap.c (grub_lower_mem, grub_upper_mem): ... to
here (new file).
* include/grub/i386/coreboot/memory.h (grub_machine_mmap_init): New
function prototype.
