Fixes#1708.
Currently involuntary preemption causes the current thread not only to
yield for a higher prioritized thread, but all other threads of its own
priority class, too.
This PR adds the function `thread_yield_higher()`, which will yield the
current thread in favor of higher prioritized functions, but not for
threads of its own priority class.
Boards now need to implement `thread_yield_higher()` instead of
`thread_yield()`, but `COREIF_NG` boards are not affected in any way.
`thread_yield()` retains its old meaning: yield for every thread that
has the same or a higher priority.
This PR does not touch the occurrences of `thread_yield()` in the periph
drivers, because the author of this PR did not look into the logic of
the various driver implementations.
Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
Finding read-before-writes statically is quite difficult. In native we
can use valgrind, but on boards without an MMU we are out of luck.
x86 has an MMU, let's use it.
If `-DDEBUG_READ_BEFORE_WRITE` was set, then pages on the heap get
initialized upon first use. If the page was read before written, then a
debug message with the virtual and physical address of the memory
location is printed, as well as the address of the offending instruction.
