net/sixlowpan: Fixed calculation of fragment size

Fragment size calculation previously failed for devices that are able to
transmit bigger layer 2 PDUs that 802.15.4 devices. This commit fixes the issue.

gnrc_ipv6_nib/nc: remove from _next_removable on remove

periph/uart: improve documentation

gnrc_netif_ieee802154: drop duplicate broadcast packets (optionally)

Change description to reflect the configurability of the serial
interface.

Also, fix the uart_mode parameter description as the UART will be
configured and not initialized.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

atmega: refactor cpu/board code and build/flash variables

To make the CI happy ;-).

sys/ps: Fixed stringification

The `_next_removable` list manages the cache-out of the neighbor cache.
However, when a neighbor cache entry is removed, it is not removed
from that list, which may lead to a segmentation fault when that list is
accessed, since the whole entry (including its list pointer) is zeroed
after removal.

With this change the entry is removed from that list accordingly before
the zeroing happens.

makefiles/toolchain: fix command -v multiple commands

drivers/bmx280: fix dead links

URLs have been changed to:

https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BMP280-DS001.pdf
https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280-DS002.pdf

)

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

Added missing string for STATUS_COND_BLOCKED, so that the shell command "ps"
no longer crashes when a thread has this state.

cpu/esp8266: removes netstats in esp wifi netdev driver

kinetis: add support for S9KEAZ128

boards/sodaq-autonomo: switch to flashing with bossac

drivers: add support for SHT2x (I2C temp and humidity sensor)

cpu/stm32_common: fix UART documentation

gnrc: nib: fix DOXYGE -> DOXYGEN typo

Don't include overridden typedefs into doxygen as otherwise,
they'll appear twice.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

Several parts were inspired by a driver written by:
  George Psimenos (gp7g14@soton.ac.uk)
  Steffen Robertz (steffen.robertz@rwth-aachen.de)

cpu/esp32: GPIO macro cleanup in periph_cpu.h

gnrc_sixlowpan_iphc: fix _compressible()

tests/gnrc_tcp_*: remove default PORT for native

Features must be provided by the board if they're actually
available on board. Other features might be provided by the
CPU.

Some grouping is also removed as it is not necessary.

Additionally, it removes unnecessary exports and cleans up
waspmote-pro toolchain variables (not needed) which are taken
from atmega_common.

Leverages common flasher (avrdude) and removes unnecessary exports.
Moreover, a reuse of serial.inc.mk is perfomed from the same
atmega_common/Makefile.include

Everything is now defined in atmega.inc.mk, following the common
RIOT-like reusability of rules and variables (e.g. cortexm.inc.mk).

Allows to use avrdude as a flashing tool in any context
(e.g. not dependent on arduino or atmega) though it only
works (AFAIK) on atmega, but I thought it's better to
have it here as we have other flashing tools.

shell_commands: adapt ping6 to original-ping-like implementation

drivers/w5100: Fixed netdev_driver_t::recv() API

When either `gnrc_sixlowpan_iphc_nhc` or `gnrc_udp` is not compiled
in `_compressible()` never returns `true`. This causes the
`dispatch` snip in `gnrc_sixlowpan_iphc_send()` to be of length 0,
meaning `dispatch->data` is `NULL`, causing possible crashes when
trying to send IPv6 packets over 6LoWPAN without NHC or UDP.