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.

mcs: tickless scheduler implementation

This changes the budget/remaining fields in scheduling contexts
to contain timer ticks, not number of abstract sel4ticks.

seL4_SchedControl_Configure now takes microseconds, not ticks.

This commit is plat-independant - the platform and arch specific
timer code follows in later commits.

mcs: Add a scheduling context object

This is the first part of the seL4 MCS. This commit:

* adds a scheduling context object. Threads without scheduling
context objects cannot be scheduled.
* replaces tcbTimeSlice with the scheduling context object
* adds seL4_SchedControl caps for each core
* adds seL4_SchedControl_Configure which allows users to configure
amount of ticks a scheduling context has, and set a core for the
scheduling context.
* adds seL4_SchedContext_Bind, Unbind and UnbindObject, which allows
a tcb to be bound to a scheduling context.

style: consistently attach return type

Add attach-return-type to astyle

style: consistently align pointer with name

Run astyle with align-pointer=name

Add CONFIG_ to syscall.c and add missing include file

Remove unneeded usages of `VISIBLE`

Using `VISIBLE` where it is not needed limits the ability for the
compiler to optimize, especially when using whole program optimizations

SELFOUR-632: implement cores non-architecture dependent structres

Add debug functionality to print the kernel entry reason and arguments at any point in the kernel

unsigned int -> word_t

Release snapshot