uncrustify: change en/disable markers

doc: add reference to RIOT paper

.gitignore: exclude results directory

gnrc_icmpv6_echo: avoid crashing when pktbuf full

This directory is created by the compile_and_test_for_board script when ran from RIOTBASE and no custom value is specified

tests: provide unittests for gnrc_sixlowpan_frag/rbuf

boards/nrf52840-mdk: initial support

Once the packet buffer is full on heavy network load, gnrc_netif_hdr_build may return NULL. In that case, the following unchecked access to hdr->data leads to a crash.

drivers/enc28j60: fix of #9784

`gnrc_sixlowpan_frag` internally derives the offset value directly
from the fragment header, so for normal usage within GNRC this
assertion is redundant, but to make the tests of `rbuf_add` 100%
water-tide I added it.

The reason I put these unittests into their own application is that I
intentionally wanted to test this module very isolated.

cpu/nrf5x: rework periph_uart driver to allow use of multiple UARTs with nrf52840

pkg/semtech-loramac: add timer calibration

This commit also change the STDIO UART to use the UARTE device type (with EasyDMA)

This requires to implement the use of EasyDMA for this kind of peripheral

gnrc_sixlowpan: stop type-finding loop when type is found

gnrc_ipv6_nib: release when not queuable on AR

Currently the loop just continues to run after a viable type is found.
In #10851 this lead to a crash of the tests, when the dependency of
`gnrc_sixlowpan` to `gnrc_ipv6` was removed.

uncrustify: add "sp_after_cast = remove"

boards/phynode-kw41z:enable support for cc811 sensor

socket_zep: fix undefined timestamp conversion behavior

When a new queue entry is tried to be allocated for a neighbor who's
address is currently tried to be resolved there was no error case
before. The packet that was tried to be put in the queue was thus not
released and stayed in the packet buffer for ever.

boards/hifive1: fix CLOCK_CORECLOCK