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.

CMake: Add KernelHardwareDebugAPIUnsupported

There are platforms/archs that don't support the Kernel Hardware Debug
API and they can now advertise that by setting
KernelHardwareDebugAPIUnsupported.

omap3: implement tickless timer driver

Update the existing omap driver to implement the new tickless api.

CMake: Invert plat config.cmake processing order

Instead of processing each platform CMake file during the arch's
config.cmake file, we process all of the platform CMake files first.
This is primarily motivated by wanting to move platform configuration
into a config file that is processed via a -C argument to the initial
build initialisation command.

Now a platform config is responsible for setting the kernel architecture
and it's own platform/arch specific config settings. Where previously a
platform was chosen in an arch specific way via either setting
KernelARMPlatform or KernelX86Sel4Arch or KernelRiscVPlatform, a
platform can now be set by KernelPlatform. In cases where a platform may
further parameterise its configuration it is free to choose its own
config options to query. Platforms that support multiple seL4
architectures should use KernelSel4Arch to query this. Platforms that
provide sub platforms such as exynos5 and subplatforms exynos5250,
exynos5410 and exynos5422 can be selected by specifying
KernelPlatform=exynos5, KernelARMPlatform=exynos5410 for example.

omap3: use declare_default_headers

- remove unused header files replaced by declare_default_headers
- move timer.h to drivers/timer/omap3430.h
- move interrupt.h to drivers/irq/omap3.h

style: use consistent styling for all cmake files

Add .cmake-format.yaml which defines custom functions with kwargs to
style nicely

serial-refactor: Refactor kernel serial drivers

Have added a drivers/serial folder to kernel, where all serial drivers
will be kept. The point is to have the the dts parsed and generate cmake
to include the right uart.c file prefixed with the compatibility.
Have removed all io.c from plat and includes from plat/config.cmake and
updated CHANGES file.

arm: use DTS to generate memory region lists

This changes all ARM platforms over to using the device tree found in
the kernel to generate memory region information.

Add a CMake based build system

This commit adds an alternate build system using CMake that operates indepenently of
the existing Kconfig+Kbuild+make based build system