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.
rpl: mute RPL

Removes all but one occurrence of printf and puts within routing (the
remaining puts is called from a debug function.
add backspace functionality in shell
Add 0x7f as backspace key in shell
native: -DNATIVE_INCLUDES if using NATIVEINCLUDES

It can be useful to know if the current compilation builds the native
board module, because e.g. then we must not include `sys/types.h`.

This diff adds `-DNATIVE_INCLUDES` to the make variable
`NATIVEINCLUDES`.
 core: Provide ssize_t in a common place

 Now you can include `kernel_types.h` if you need the `ssize_t`, without
 facing problems due to the terrible MSP toolchain.
cpu/nrf51822: fixed gpio driver
boards: added LED macro for pca1000 & yunjia-nrf
Merge pull request #1698 from Kijewski/ssize_t-for-all

core: Provide ssize_t in a common place
sys/net/ccn_lite/README.md change wording + add ACM ICN reference

sys/net/ccn_lite/README.md fix encoding
pkg/Makefile.{http,git}: Minor issues resolved.

The were two recipes for clean. One of them is now "dist-clean"
make: 1571 fixup: dont create existing directories

This fixes an error which was introduced by commit
346313bf07304c5636739d028c7b70acb4b507f9

The timestamp of directories is updated when a file inside a directory
is changed.
Therefore, make decides a target needs to be rebuilt, whenever that
target depends on its parent directory, because the directory is
always newer than the file inside.

http://www.gnu.org/savannah-checkouts/gnu/make/manual/html_node/Prerequisite-Types.html

    Occasionally, however, you have a situation where you want to
    impose a specific ordering on the rules to be invoked without
    forcing the target to be updated if one of those rules is
    executed. In that case, you want to define order-only
    prerequisites. Order-only prerequisites can be specified by
    placing a pipe symbol (|) in the prerequisites list: any
    prerequisites to the left of the pipe symbol are normal; any
    prerequisites to the right are order-only:

         targets : normal-prerequisites | order-only-prerequisites
make: optimize archive creation/modification

Only add/replace changed members to the archive.
Merge pull request #1462 from backenklee/smallPR

pkg/Makefile.http: There were two recipes for clean.
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.
Merge pull request #1725 from LudwigOrtmann/dont-make-alot

make: only depend on directories that don't exist
Merge pull request #1528 from OlegHahm/silent_rpl

rpl: mute RPL
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