shell: Add doxygen

Closes #577.
native/make: add missing target
Merge pull request #1037 from Kijewski/issue-577

shell: Add doxygen
Merge pull request #1042 from LudwigOrtmann/native_make-all-debug-target

native/make: add missing target
tests: fix test_hwtimer_spin

Actually test hwtimer_spin, not hwtimer_arch*

`hwtimer_wait` calls `hwtimer_spin` only for delays `<= 6`, that's why a
pure test_hwtimer_spin test should not pass values `>6`.
Also decrement delay in steps of 1.
tests: add test_hwtimer_wait

It contains the old test functionality of test_hwtimer_spin.