tests: introduce dedicated `gnrc_rpl_srh` test

tests/gnrc_ipv6_ext: port to scapy

frdm-k64f: Add LPTMR configuration

cpu/esp: esp_now_netdev fixes and optimizations

drivers/pulse_counter: refactor the prototypes of read funcs

cpu/esp8266: doc extended

Documentation is generated by doxygen from doc.txt

sock_dns: fix out-of-bound errors

Fixes #10739

drivers/cc2420: Implemented missing drop case

tests/gnrc_udp: Fixed typo in Makefile

cortexm.inc.mk: detect 'arm-none-eabi-gcc' from ubuntu bionic

arm-gcc version from ubuntu bionic repository is not supported in RIOT.
Both when building with `gnu` and `llvm`.

    arm-none-eabi-gcc --version
    arm-none-eabi-gcc (15:6.3.1+svn253039-1build1) 6.3.1 20170620

So detect this version and print an error if found.

The check is done on the building machine and not on the host when building in
docker.

The error can be disabled when building with WERROR=0.

net/gnrc/netif: Added CC1xxx adaption layer

assert was added intentionally for debugging purposes. For released version it is enough that function returns.

This adaption layer is intended to be used by a rewrite of the CC110x driver and
a new CC1200 driver that is currently being developed for RIOT.

gnrc_netif_*: use the `gnrc_netif_t::pid` member instead of the pid of the current thread in netif header

drivers/ds18: refactor functions with const dev pointer

tests: add nanocoap CLI app for client and server testing

core: remove redundant casts

drivers/tps6274x: Initial support for the step-down converter

Use the `gnrc_netif_t::pid` member instead of the pid of the current thread when generating the the `gnrc_netif_hdr` in `gnrc_netif_ieee802154::_recv` function.

Use the `gnrc_netif_t::pid` member instead of the pid of the current thread when generating the the `gnrc_netif_hdr` in `gnrc_netif_ethernet::_recv` function.

drivers/tps6274x: initial support step-down converter
tests/driver_tps6274x: Added testcase for the step-down converter
boards/jiminy-mega256rfr2: Added TPS6274x config

This change makes it more clear that the dev is modified in some of the
read functions. Only pulse_counter_read_without_reset gets a const
pointer argument.

Also, use pulse_counter_t pointers instead of void pointers where
possible.

boards/esp32-wroom-32: configure saul button as inverted