kernel/arch: Move early init/boot code out of init/kernel headers

Cleanup init.c code and move early boot code into arch/ and make it
accessible outside of the boot process/kernel.

All of this code is not related to the 'kernel' and is mostly used
within the architecture boot / setup process.

The way it was done, some soc code was including kernel_internal.h
directly, which shouldn't be done.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>