rtc_os: Added chaintask(TASKX)

    The chaintask is a bad fusion of activate task and terminate task
like in dbz movie:

http://images3.wikia.nocookie.net/__cb20111027152718/dragonball/images/4/47/VekuHidesFromJanemba.png

Basically it disables the interrupts and reset the task structure
values, then sets ready the required task, re-enables the interrupts.

Then moves the SP to the start of the active task and calls the RTC
instruction.

Signed-off-by: Emiliano Gibran Gonzalez Perez <haibrayn@hotmail.com>