This adds a mechanism for modules to declare requirements on the thread
stack size in their `Makefile.dep` and let the build system then
override the default stack size, if any requirements are declared.
The motivation is to allow multiple modules to have special requirements
without causing conflicts, as just adding the following to their
`Makefile.include` would do:
```Makefile
CFLAGS += -DEVENT_THREAD_MEDIUM_STACKSIZE=<MAGIC_NUMBER>
```
Instead, the new mechanism would work by having them both declare in
their `Makefile.dep`:
```Makefile
EVENT_THREAD_MEDIUM_STACKSIZE_MIN +=
```
The build system then picks the maximum number in
`EVENT_THREAD_MEDIUM_STACKSIZE_MIN` and exposes this as stack size, if
any module did declare a minimum requirement.
Co-authored-by: benpicco <benpicco@googlemail.com>
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.
