In many places we needlessly use `sched_active_thread->pid` whilst we
already have `sched_active_pid` with the same value, and one less
indirection.
`thread_getpid()` is made `static inline` so that there is no penalty in
using this function over accessing `sched_active_pid` directly.
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.
The `arm_common` syscalls implement `_gettimeofday()` for the benefit of
newlib. The syscall call only work if the `rtc` module or the `vtimer`
modules is transcluded. If neither module is used, a warning is printed
by means of `#warning`.
This warning is useless if the user does not invoke `gettimeofday()`.
Further this warning prevents the use of `-Werror`.
This PR puts the function in its own file, that will only be linked if
it was used. And if the function was used, then a link time error occur
if neither `rtc` nor `vtimer` was transcluded.
RIOT's gettimeofday needs to be called from syscalls.c to assure that gcc actually links it.
If an RTC if available and enabled it will be used instead.
