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: Configuration options for different multiboot headers

This allows for disabling a particular multiboot version if there are problems
using it with a particular loader.

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.

Fix licenses (the rest)

x86: Expose multiboot video mode selection

x86: Remove duplication of the multiboot header