Rework SPI periph driver to use proper RIOT GPIO API functions.
    Also cleanup header files by using vendor defines and remove
    obsolete code. Further, adapt board config accordingly.

    Introduces a define to inidicate an unused function parameter.

makefiles/buildtests.inc.mk: allow overwriting make output redirection

gnrc_ipv6_nib: fallback to SLAAC if ARO is ignored by upstream

make: add APPDIR to info-build target

    While info-build already provides lots of useful information
    it does not print the application source dir. However this
    might be useful for debugging and logging, hence its added
    to the output.

tests: remove echo=False

drivers/sdcard_spi: fix uint64_t cast location

Linux doesn't have ARO support at the moment so this is a workaround to
try to speak 6Lo-ND while still being able to do DAD with a border
router that doesn't.

When compiling `tests/drivers_sdcard_spi` with `avr-gcc: avr-gcc (GCC) 6.4.0`
it detected this error

    RIOT/drivers/sdcard_spi/sdcard_spi.c:1012:72:
    error: result of '512 << 10' requires 21 bits to represent, but 'int' only has 16 bits [-Werror=shift-overflow=]
    return (card->csd.v2.C_SIZE + 1) * (uint64_t)(SD_HC_BLOCK_SIZE << 10);

When running tests, I often needed to modify `buildtests.inc.mk` to see the
actual errors/standard output.
This allows overwriting the default redirection of both to `/dev/null`.
It is low level and directly given to the command execution.

As it is interpreted by make, it can even be overwritten using build system
variables:

    'BUILDTEST_MAKE_REDIRECT=>/tmp/buildtest.$${board}.out 2>&1'

boards/b-l475e-iot01a: configure second I2C and on-board sensors (lis3mdl, lsm6dsl and hts221)

kinetis: uart: fix unused-function warning

When compiling with LLVM (should also be seen `-Wunused-function` in
GCC), I get an error for `frdm-k22f`. This should fix that.

net/gcoap: Make references to coap_resource_t all const in gcoap

tests/gnrc_netif: enable CI tests

gnrc_ipv6_nib: make automatic NDP packet emission (NS/RS) configurable

drivers/hdc1000: change renew_interval to a parameter

This test doesn't test NDP behavior so emissions of NDP neighbor and
router solicitations is more harming than helpful. Because of that I
disabled it for this test.

boards/nucleo-f091rc: add i2c configuration

cpu/cc2538: generalise SPI clock configuration

doc: move wiki to Doxygen documentation (second attempt)

boards/nucleo-f070: add missing periph_i2c feature

    The SPI bus frequency/clock is calculated relative to the MCUs
    core clock. Currently all boards use the default 32MHz, hence
    prescaler settings for SPI are all the same. This PR moves the
    default config for 32MHz to the CPU and allows to be overriden
    by board config if needed.

examples: dtls-echo: remove unneeded addressing operator

Add support for the SODAQ ONE board

tests/periph_uart: add automated testing script

Also add sodaq-one to the boards with insufficient memory

The following features were tested (briefly):
* UART (the first)
* xtimer is working (it's the same code as in samr21)