Kernel tests are done by replacing the implementation of an otherwise
empty function 'Kernel::test' that gets called once at the primary CPU
as soon as all kernel initialization is done. To achieve this, the test
binary that implements 'Kernel::test' must be linked against the core
lib and must then replace the core binary when composing the boot image.
The latter can be done conveniently in a run script by setting the new
argument 'core_type' of the function 'build_boot_image' to the falue
'test'. If no kernel test is needed the argument does not have to be
given - it is set to 'core' by default which results in a "normal"
Genode image.
ref #1225
