replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

x86: add framebuffer info of MBI2 via boot_info

x86: evaluate ACPI RSDP v1 multiboot 2 information

x86: Avoid zero sized array extension

Zero sized arrays are not a C standard extension supported by the C parser. This changes
to the older C90 style for variable length arrays of declaring an array of length 1, as
the C parser also does not suppor the C99 flexible array members.

x86: Add missing license tag

x86: Support for mulitboot2 compatible booting

Declares to a bootloader that we hare multiboot2 compliant and provides an additional
try_boot_sys_mbi2 function for processing a multiboot2 information header.