rpl: mute RPL

Removes all but one occurrence of printf and puts within routing (the
remaining puts is called from a debug function.
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 #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