net/nanocoap: verify simple reply buffer

net/netreg: fix static netreg init macros

boards/arduino*: Added/refactored params for W5100

sys/base64: Add size estimation functions

gnrc_ipv6_nib: activate SLAAC on border router

net/nanocoap: validate option length

cpu/atmega_common: do not export LINKFLAGS

The space calculation must use the same length as the eventual
return value.

gnrc_sixlowpan_frag: release for fragment too big error case

makefiles/vars.inc.mk: do not export LINKFLAGS

dist/tools/compile_and_test_for_board: add basic checks for script

This prevent evaluating `LINKFLAGS` when not needed and when building
in docker so does not produce errors if `avr-ld` is not installed.

```
BUILD_IN_DOCKER=1 BOARD=arduino-mega2560  make --no-print-directory -C examples/hello-world/ clean
/srv/ilab-builds/workspace/git/riot_master/makefiles/toolchain/gnu.inc.mk:18: objcopy not found. Hex file will not be created.
/bin/sh: 1: avr-ld: not found
```

It removes the `/bin/sh: 1: avr-ld: not found`

It is only used by `Makefile.include` and should not be used by sub-make
instances.

This is required to prevent evaluating `LINKFLAGS` when not needed and a
required step to not evaluate it on the host with for example `avr-ld`
when building in docker.

I checked usage with:

    git grep -e '(LINKFLAGS)' -e '{LINKFLAGS}'

Packages may be using it but `LINKFLAGS` is a RIOT way of naming
things and even if `generate-xcompile-toolchain` uses `LINK` it does not
use `LINKFLAGS`.

Provide some notes on how to perform basic checks on the script using tox

tox.ini configures 3 checks on the python script: doctest (via pytest), pylint and flake8

The fragment is released for all other error cases, just not this one.

Otherwise it conflicts with pytest automatic parsing which tries to execute all classes with name beginning with Test

cpu/kinetis/rtt: clear TPR when writing TSR

net/nanocoap: add debug message for server

sys/crypto/aes: avoid UB by explicit unsigned integer promotion

cpu/nrf5x: handle multiple exti pins in gpio driver

Each pin is associated to a given GPIOTE channel

gnrc_sixlowpan: document submodules according to #8511

- ATmega based boards: Added parameter for W5100 to board_common.h
- Arduino Due based boards: Use board.h instead of providing w5100_params.h

gnrc_lwmac: enable radio duty-cycle printing.

net/gcoap: make options buf macros configurable

examples/bindist: cleanup and fix formatting in README

gnrc_mac: add timeout module.

dist/tools/compile_and_test_for_board: add compile and test script

cpu/saml1x: add support for SAML10 and SAML11 MCUs (Cortex-M23)

cpu/atmega_common: refactor AVR libc code into module

cpu/esp8266: change of ETS task handling