[AVR] Add preliminary tickless support to HAL

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@7101 35acf78f-673a-0410-8e92-d51de3d6d3f4
[AVR] Enable tickless usage for NIL

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@7102 35acf78f-673a-0410-8e92-d51de3d6d3f4
[AVR] Use tickless mode in ArduinoMega NIL demo

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@7103 35acf78f-673a-0410-8e92-d51de3d6d3f4