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.