prevent rounding 0 if HWTIMER_SPEED > 1000000L

The current macros in hwtimer.h expect HWTIMER_SPEED to be < 1000000L, otherwise integer arithmetic will round the result down to 0.
Add a case to prevent that.
doc: added missing closing doxygen brackets
doc: use @brief for single defines
doc: added missing titles
doc: fixed typos and other misspellings
doc: use @false instead of 0
doc: use doxygen li tags
doc: add to group instead of redefining
doc: fix typo in UDP header
doc: remove doxygen duplication
doc: removed broken doxygen struct tags
doc: removed superfluous (and broken) @file params
doc: fixed broken param descriptions
Add UBJSON library
tests: add UBJSON unittests
core/thread: use disableIRQ/restoreIRQ
Merge pull request #1659 from Kijewski/ubjson

sys: add  Universal Binary JSON module (UBJSON)
core: undefining ENABLE_DEBUG is no longer required
debug: removed superfluous additional debug flag
Merge pull request #2130 from OlegHahm/debug_cleanup

Debug cleanup