Pull up following revision(s) [adapted, via patch]
(requested by manu in ticket #1382):

sys/arch/i386/include/multiboot2.h: revision 1.1
sys/arch/i386/stand/lib/exec.c: revision 1.74
sys/arch/i386/stand/efiboot/efiboot.c: revision 1.11
sys/arch/i386/stand/efiboot/bootia32/multiboot32.S: revision 1.1
sys/arch/i386/stand/lib/biosdisk.c: revision 1.52
sys/arch/i386/stand/efiboot/bootx64/efibootx64.c: revision 1.5
distrib/sets/lists/comp/md.amd64: revision 1.273
sys/arch/i386/stand/efiboot/eficons.c: revision 1.7
sys/arch/i386/stand/efiboot/bootia32/efibootia32.c: revision 1.5
distrib/sets/lists/comp/md.i386: revision 1.191
sys/arch/i386/stand/lib/libi386.h: revision 1.45
sys/arch/i386/stand/lib/bootinfo_memmap.c: revision 1.6
sys/arch/i386/stand/lib/pread.c: revision 1.8
sys/arch/i386/stand/efiboot/efiboot.h: revision 1.10
sys/arch/i386/stand/efiboot/bootia32/Makefile: revision 1.3
sys/arch/i386/stand/lib/Makefile: revision 1.47
sys/arch/i386/stand/efiboot/bootx64/Makefile: revision 1.2
sys/arch/i386/stand/lib/multiboot.S: revision 1.3
sys/arch/i386/stand/efiboot/bootx64/multiboot64.S: revision 1.1
sys/arch/i386/stand/efiboot/efimemory.c: revision 1.9
sys/arch/i386/stand/efiboot/boot.c: revision 1.16
sys/arch/i386/include/Makefile: revision 1.50
sys/arch/i386/stand/efiboot/Makefile.efiboot: revision 1.16
share/man/man8/man8.x86/boot.8: revision 1.20
sys/arch/i386/stand/boot/Makefile.boot: revision 1.73
sys/arch/i386/stand/pxeboot/Makefile: revision 1.26
sys/arch/i386/stand/lib/exec_multiboot1.c: revision 1.1
sys/arch/i386/stand/netboot/Makefile.netboot: revision 1.11
sys/arch/i386/stand/lib/exec_multiboot2.c: revision 1.1
sys/arch/i386/stand/lib/biosdisk.h: revision 1.12
sys/arch/i386/stand/lib/exec_multiboot2.c: revision 1.2
sys/arch/i386/stand/dosboot/Makefile: revision 1.32
sys/external/bsd/gnu-efi/dist/inc/efiapi.h (apply patch)

Add multiboot 2 support to x86 bootloaders

multiboot 2 is required to boot Xen on an EFI system.

This also require a kernel patch for properly discovering
the ACPI RSDP, which is available after 20190912, in
src/sys/arch/x86/acpi/acpi_machdep.c 1.26-1.28

There are a few missing bit in this multiboot 2 implementation
(which are unused by Xen):
- Header tags Address, Freambuffer, and Relocatable are ignored
- Tags APM and Network are not provided
- Tags ACPI old and ACP new are only provided for ACPI boot
- Tag boot device does not provides the subpart (BSD disklabel partition)

Notes:
- multiboot2 is disabled in dosboot, otherwise the binary
  gets too big and build fails.
- in src/sys/arch/i386/stand/efiboot, consinit() is renamed
  as efi_consinit() to avoid prototype conflicts in src/sys/sys/systm.h

Remove debug define.

It remained there unseen because it was misspelled!

Remove obsoeolete BUGS note that UEFI bootloader does not support multiboot

Add new multiboot2.h header.

(martin)