doc: added documentation to the global init defs

Documented the global init automaters:

	* subsys_init
	* module_init
	* device_init

Signed-off-by: Michel Megens <dev@michelmegens.net>
doc: init: documented the general system init.

Added documentation for the ETA/OS kernel initialisation. These
documents are found in the ETA/OS initialisation module.

Signed-off-by: Michel Megens <dev@michelmegens.net>