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.

ARM: optionally pass bootloader DTB to userspace

We now support receiving a DTB from the ELF loader and passing it on
to userspace in extra bootinfo. We still support booting without a DTB,
though - the device tree address is set to zero in the boot code and no
extra bootinfo region is provided.

style: set code width to 120

Remove assumption on 32-bit memory offsets

On 64-bit platforms physical address could be >2^32 offset from a virtual address. This
changes offsets to match the word size of the target architecture

ARM: Map the initial thread's IPC buffer and boot info as non-executable.

Note that this also required some irrelevant x86 changes to match function
arguments.

Release snapshot