net/gcoap: Refine Observe re-registration

makefiles/vars.inc.mk: fixup indentation

core/mbox: doc typo fixes

drivers/at: make OK and ERROR replies configurable

boards/stm32-based: use shared configuration snippets

tools: allow static checks to be run from any directory

tools/doccheck: adapt script to run on macOS

tlsf: add custom walker to get total free / used sizes

all: fix all undefined doxygen groups

cpu/saml21: Fix possibly uninitialized variable in pm.c.

gnrc_tcp: Set src addr in outgoing packets by tcp layer.

This test is intended to collect statistics about the runtime delays in
the periph_timer implementation. This tool is mainly intended to detect
problems in the low level driver implementation which can be difficult
to detect from higher level systems such as xtimer.

tools/flake8: fix regex in check script

native: Enforce safe strncpy usage in tap device setup

net/nanocoap: Build message with coap_pkt_t

Includes string and uint options.

shell/ccnl: use new CS add method