fixed Wformat warnings
sys: rename ringbuffer functions

Closes #1011.
Merge pull request #1092 from BytesGalore/fix_print_format_warnings_test_thread_msg

test:test_thread_msg fixed Wformat warnings
core: documentation: updated, improved, and completed doxygen comments
core: documentation: fixed bitarithm documentation
core: documentation: replaced task w/ thread
core: documentation: fixed typo in license header
documentation: extend description for CONST attribute
bitarithm: remove unused macros
documentation: fixed doxygen for schedstats
Merge pull request #1099 from Kijewski/issue-1011

sys: rename ringbuffer functions
arm_common: no needless `_gettimeofday()` warning

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.
Merge pull request #1097 from Kijewski/gettimeofday-warning

arm_common: no needless `_gettimeofday()` warning
Merge pull request #983 from OlegHahm/core_doxygen_update

core: documentation: updated, improved, and completed doxygen comments
drivers: fixes to the low-level GPIO driver

- fixed doxygen comments
- added GPIO_UNDEFINED value
- added return values to all functions
Merge pull request #971 from haukepetersen/periph_gpio_fix

drivers: fixes to the low-level GPIO driver
Introduce macro to define files that should be deleted on make clean
Move .elf file to it's own target
Make directory creation in object compilation more makey