Parametric scheduler approach.

Parametric scheduler approach.

Define distinct functions for each implemented scheduler class. This is
a first step toward the "plugin-scheduler" approach.

In this way we can also get rid of mtask.c.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@3240 38d2e660-2303-0410-9eaa-f027e97ec537