tests: let license checker check headers, too
make: don't call mkdir -p a lot
make: use $@ and $<
make: call ranlib
make: don't invoke wildcard repeatedly
make: easier to read object targets
make: centralize wget/curl & unzip/7z feature test

With many open PRs that could benefit from loading SDKs when needed,
instead adding vast amounts of code to RIOTs master, this PR provides
the "functions" `$(DOWNLOAD_TO_STDOUT)`, `$(DOWNLOAD_TO_FILE)`, and
`$(UNZIP_HERE)`.

The first "function" takes one argument, the URL from where to download
the content. It is then piped to stdout. To be used e.g. with `tar xz`.

The second "function" taken two arguments, the destination file name,
and the source URL. If the previous invocation was interrupted, then the
download gets continued, if possible.

The last "function" takes one argument, the source ZIP file. The file
gets extracted into the cwd, so best use this "function" with
`cd $(SOME_WHERE) &&`.

The clumsy name `$(UNZIP_HERE)` is taken because the program "unzip"
takes the environment variable `UNZIP` as the source file, even if
another file name was given on the command line. The rationale for that
is that the hackers of "unzip" hate their users. Also they sacrifice
hamsters to Satan.
tests: limit concurrency level

Some Travis CI machines have 32 CPUs. This sets our concurrency level to 33.
Travis CI kills our buildtest for obvious reasons.

This PR limits the concurrency level to 8 on Travis CI.
cpu/sam3x8e: added random number generator driver
boards/arduino-due: enabled RNG driver
test/periph_random: added test for RNG driver
Merge pull request #1593 from Kijewski/nproc-max

tests: limit concurrency level
Merge pull request #1294 from haukepetersen/add_arduinodue_rng_driver

cpu: Add random number generator driver for the SAM3X8E
Merge pull request #1431 from Kijewski/wget-for-everyone-woooohooooo

make: centralize wget/curl & unzip/7z feature test
Merge pull request #1571 from Kijewski/make-mkdir-once

make: cleanup Makefile.base
periph_driver: introduce a RTT interface
periph:rtc: add missing argument for callback
Merge pull request #1595 from thomaseichinger/fix_rtc

periph:rtc: add missing argument for callback
Merge pull request #960 from thomaseichinger/rtc_interface

periph_drivers: Proposal for a low-level RTT driver interface
Merge pull request #1531 from LudwigOrtmann/header_license_headers

tests: let license checker check headers, too