sys/net/loramac: add device class enum

slipdev: simplify and solidify byte-unstuffing

This simplifies and solidifies the reversal of SLIP's byte-stuffing
(aka byte-unstuffing ;-)) by

1. Using `tsrb` instead of `ringbuffer`: there are two actors. The ISR
   and the device's event handler.
2. Moving the byte-unstuffing from the UART RX-handler (i.e. the ISR)
   to the device's receive function (potentially not the ISR)
3. Removing the `pktfifo` member. The current number of bytes in the
   ringbuffer is returned for `recv(data = NULL, len = 0)`. If that is
   more than the packet contains (due to the byte stuffing it most
   likely will be) the packet is reallocated in GNRC anyway.

cpu/nrf52: add i2c driver and configs

cpu/stm32l1: fix ADC resolution register

boards: adds support for Nordic Thingy:52

tests/sx127x: adapt to driver interface change

 gnrc_netif: add nrfmin IID support

This adapts automatically when a variable type changes, which is easy
to forget to change the correspondig netdev->set/get call.

gnrc_ipv6_nib: fix nrfmin module check

Adds a `nrfmin` case to IID generation, so an IID can be generated
from a `nrfmin` device's (short) address (the long address is just
4-time repetition of the short address).

Moves setting of addresses of length lesser than 3 to a function that
initializes the first 5 byte of the IID with `00:00:00:ff:fe` and then
fills up the rest with zeroes and the given short address.

The module is called `nrfmin` not `netdev_nrfmin`. As such this compile
path was never included, even if the `nrfmin` module was present.

drivers/sx127x: add static keyword to inline

dist/tools/headerguards: fix flake8 issues

tests/rng/tests: fix flake8 F401 issue

dist/tools/mkconstfs: fix various flake8 issues

sys/net/loramac: rename network session key

tests: fix some makefiles

sys/crypto/ccm: fix docstring value for 'nonce_len'

tests: add program for priority inversion problem

toolchains: print git version

boards/ACD52832: add new NRF52 based development board

F401 'pexpect' imported but unused