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.

style: consistently align pointer with name

Run astyle with align-pointer=name

Add unverified_compile_assert

This version of compile_assert is not expanded during verification builds and
can be used in cases where you would like an assertion that contains syntax
that is not supported by the C parser

Add missing include in assert.h

s/DEBUG/CONFIG_DEBUG_BUILD/

DEBUG definition is not supposed to be used in the kernel, rather CONFIG_DEBUG_BUILD,
which can be toggled separately to user notion of DEBUG

Always have compile_assert enabled

A compile assert has no runtime or image size overhead and so there
is no reason to not have them enabled

This commit addresses SELFOUR-289

Release snapshot