tests/libfixmath_unittests: fix for iotlab-m3 and samr21-xpro

cpu: ldscript memory attribute corrections

cpu/stm32f4: add support for STM32F423 line

On iotlab-m3 test duration was 80 seconds and 250 on samr21.

The test fix16_str_unittests uses snprintf("%0.5f") to compare to fix16_to_str.

pkg/semtech_loramac: handle non OK mcps confirm messages

pkg/semtech-loramac: add link check support

gcoap: make message queue size configurable

kinetis: Memory segment attribute fixes

tools/flake8: fix shellcheck issues

pkg/qDSA: disble arm asm version for cortexm > 0plus

vectors and flashsec memory segments will not be considered for orphan
sections if rx is only given for the rom segment.

sys/arduino: use RIOTTOOLS variable

tests/thread_race: add test for race conditions

gnrc_rpl: remove route before updating it

The "new" forwarding table does not update an old route but just adds
another as long as it is not *exactly* the same. However, the RPL
adaptation missed to remove the old route so RPL got easily confused
about where it actually needed to send packets.

pkg: use RIOTTOOLS variable

tests: use RIOTTOOLS variable