cpu/stm32l4: add adc implementation

pseudomodules: allow defining them in pkg/PKG/Makefile.include

Officially allow packages to define pseudomodules in their Makefile.include as
done in the libcose package.

net: add Asymcute, an asynchronous MQTT-SN client implementation

Makefile.include: fully define BASELIBS before using its value

mosquitto_rsmb: add MQTT listener to config

This helped me a lot while testing #9464 to interact with the broker
using the mosquitto shell command clients.

gnrc_ipv6_nib: Add capability to handle and send RDNSS option

sock_util: add posix dependency

core/byteorder: fix naming for htobebufs/bebuftohs functions

`sock_util` includes the `arpa/inet.h` header so it has `posix` as a
dependency to include the `sys/posix/include` include path.

drivers/at: at out-of-band data support for at commands parser

tests: cpu_efm32_features: add test application

Add SRAM based PRNG seeder

tests/monocypher: Increase stack space

tools: add mosquitto.rsmb MQTT-SN broker

Improving TinyDTLS package and dtls-echo example

Due to all the changes, this is basically a new version for this
example. The main benefit is the use of sock_udp but also the client
side is now more robust and reliable.

The parameters required for the PSK and ECC (e.g. keys) modes are moved
to an unique header.

The integration of TinyDTLS for RIOT has been merged into the
development branch of the official repository.

It will be officially available on the master branch in the next
release of tinyDTLS.

Other minor updates:
- Support for the RIOT's memarray
- tinydtls/sha2 is removed from the compilation for giving priority
to RIOT's sha2 functions.