time: doc: added cs_get_diff and tm_update_source

Added documentation to

	* cs_get_diff
	* tm_update_source

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

* Added documentation to the ATmega AVR module about the system
  initialisation. The first moments of power on are described in detail.

* added a most minimal application example to the ETA/OS kernel
  module documentation.

Signed-off-by: Michel Megens <dev@michelmegens.net>
ETA/OS 1.0.0

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