core: refactor sched_run

`sched_run()` was cluttered. Many individual changes were done without a
proper refactoring.
Merge pull request #1841 from Kijewski/sched_yield-result

 core: refactor sched_run
lint: fix unassignedVariable warnings in stm32f1
lint: fix unusedVariable warnings

SQUASH ME: into unusedVariable (annotated)

SQUASH ME: into unusedVariable (II)

SQUASH ME: into unreadVariable (II)
lint: fix redundantAssignment warnings

SQUASH ME: redundantAssignment (remove FAT stuff)

SQUASH ME: into redundantAssignment unnecessary supp
lint: fix unreadVariable warnings
lint: fix variableScope warnings
lint: fix invalidPrintfArgType_sint  warning
lint: fix uselessAssignmentPtrArg
lint: fix warnings of possible nullPointer dereferences
lint: fix nullPointer errors
lint: fix syntaxError warning
cpu: stm32f1 spi: fix typos
lint: suppress warning about obsoleteFuntionsasctime
lint: suppress unusedStructMember in vendor files
lint: fix redundantCopy in udp code
lint: fix va_end_missing in cpu/native/syscalls.c
lint: fix uninitstring in sixlowdriver