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.

Remove symlinked libsel4 files from include dir

These files can be included normally using libsel4 include paths. This
removes situations where the same file is available under different
include paths due to symlinking into different directory structures.

SELFOUR-114: remove bootinfo.h duplication

Change bootinfo to have generic slot region for paging structures

Current bootinfo defines two slot regions for paging structures,
one for page directories and one for page tables. This does not
easily generalise for N levels of paging structures. This change
uses a single region that has a known order of objects for however
many levels exist

unsigned int -> word_t

Rename uint32_t -> word_t in any relevant places

ia32: Large commit that implements PAE paging for x86

This commit involves the PAE paging itself, refactoring the
vspace code to extra what is common between PAE and 32bit paging,
as well as some renaming to not call the root paging structure
a PD

trivial: Standardise on 32-bit unsigned variables for domain values.

Domains are unnecessarily treated explicitly as 8-bit values within boot info.
Though there are existing proof constraints that limit the maximum domain value
to 8 bits, most of the code would permit domain values up to 32 bits. The
maximum value is unnecessarily constrained in boot info, a restriction which
this commit removes.

Closes VER-341

Release snapshot