unittests: make: Fix unittests for branch changes

Changing between branches in git typically leaves directories empty and
does not remove them. If you have different unittests in different
branches this leads to an error in building them, since the current
configuration expects a Makefile in all `tests-*` directory. This change
configures Make to search for a Makefile in `tests-*` rather than just
assuming that there is one.
make: del no-delete-null-pointer-checks for clang++

This PR enables the use of clang++ to compile C++ projects by adding
`CXXUWFLAGS += -fno-delete-null-pointer-checks`, if `${CC}` understands
`-fno-delete-null-pointer-checks` but `${CXX}` does not.
core: Provide functions for different byte orders

Rationale: see #1586.
tests: Add unittests for byteorder.h
tests: fix bad copy+paste in tests-core.h
reverted forcing travis to install `gcc-arm-none-eabi=4-*`
Merge pull request #1742 from BytesGalore/revert_workaround_for_gcc_arm_none

Travis: reverted forcing travis to install `gcc-arm-none-eabi=4-*`
travis: remove comment obsoleted by #1742
Merge pull request #1743 from N8Fear/travis

travis: remove comment obsoleted by #1742
Merge pull request #1645 from authmillenon/unittests-empty-dirs

unittests: make: Fix unittests for branch changes
tests: fix mutex_unlock_and_sleep pids
Merge pull request #1744 from N8Fear/fix-mutex_unlock_and_sleep

tests: fix mutex_unlock_and_sleep pids
make: Allow for include of modules outside of the RIOTBASE path
sixlowpan: fixed net_if counter

This is a fixup for 11254577eb01178b4ca90fe6383987cc0721fca6 which
caused this function to always return a positive number, when any
interfaces was configured.
drivers/periph/gpio: increase possible gpio number
Merge pull request #1748 from LudwigOrtmann/raisingthebar

drivers/periph/gpio: increase possible gpio number
atmega_common: add ENOTSUP and ECANCELED to errno.h
Merge pull request #1745 from OlegHahm/is_our_address_fixup

sixlowpan: fixed net_if counter
introduce HWTIMER_SPIN_BARRIER (API change)

Boards should define HWTIMER_SPIN_BARRIER that is used to decide
whether it makes sense to set a timer and yield or call hwtimer_spin
instead.
Used by `core/hwtimer.c` and `sys/vtimer/vtimer.c`.
A default value is provided and a warning is printed when it is used.
core/hwtimer: clean up header

- break/shorten overlong lines
- improve grammar