Allow using `event_loop_multi()` to handle event queues of multiple priorities
in an single thread. In the extreme case, all three event queues are handled
by a single thread (thus saving two stacks). This comes for the price of
increased worst case latency, as already running event handlers will no longer
be preempted by higher priority events.
With this, all three event queue priorities are always provided. Using modules,
the old behavior of one thread per event queue can be restored for better worst
case latency at the expense of additional thread size.
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
When the expected ROM overhead of a function is bigger than the actual function,
it is better to provide the function as static inline function in the header.
Added `event_wait_multi()` that takes an array of event queues rather than
a single event queues. The queue with the lowest index will have the highest
priority.
Unstructured static initializer like { 0 } lead to compilation errors on ESP8266, MSP430 and MIPS. Error messages are:
error: (near initialization for 'queue.event_list') [-Werror=missing-braces]
error: (near initialization for 'queue.waiter') [-Werror=missing-field-initializers]
This change fixes the compilation problem.
