Decide between HRTT or SRTT on FlexPRET platform

[magnmaeh]

Unlike other platforms, FlexPRET supports hard real-time threads (HRTTs) and soft real-time threads (SRTTs). When creating a thread using FlexPRET's API, one has a choice on whether to use a HRTT or SRTT. Usually it is the programmer's task to decide, but in lf_flexpret_support.c, the runtime must do it.

Currently, all threads are HRTTs. But the LF runtime could potentially select between HRTT or SRTT based on the importance of the task. E.g., a reaction that prints to stdout might not need to be HRTT, but one that handles physical actions from interrupts maybe should be HRTT.

Maybe reactors could be extended to have some kind of priority/importance level. Or maybe some label/additional information could be passed to the reactors that is only available when the target platform is FlexPRET. E.g., maybe something like this:

@FlexPRET(SRTT)
reactor Print {
    // ...
}

@FlexPRET(HRTT)
reactor Actuator {
    // ...
}