mainpage.md: fix dead link and 3 typos

 murdock: add selected cortex-m boards to LLVM build

tests/libc_newlib: fix pointers comparison for llvm

With llvm and samr21-xpro, I could not directly do 'printf == iprintf'.
But doing `(printf - iprintf) == 0` correctly checked if they are equal.

Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>

dist/cppcheck: fix missing cppcheck-suppression reasons

dist/tools: extend the print_toolchain_version script

This was found by cppcheck 1.6x used in Travis-CI, however this
seems to be a false positive, as it is not found by newer versions
of cppcheck.

Cppcheck was (correctly) warning here that concat to strings might
result in buffer overflow because the terminating `\0` was not considered.
This is fixed here, making the cppcheck suppression also obsolete.

Extend the cppcheck suppression example to show that each suppression
should have a reason describing the intentional suppression of a
cppcheck warning or error.

Adding and correcting description/rational on why certain cppcheck
warnings or errors are intentionally suppressed.

tests/gnrc_sixlowpan: fix stacksize of dummy device

Add output to print_toolchain_version script to include info
on the operating system and its kernel version.

This adds openocd, python, python2, and python3 to the helper
script print_toolchain_versions.sh.

Currently version information of commands and tools is only parsed
from STDOUT, however some tools like openocd print version info
to STDERR only. This commits adds parsing of STDERR for version
infos if STDOUT does not contain such info.

at driver: allow using same buffer for command and response in at_send_cmd_get_lines()

This was already too small in 6c2510dc so it should have been properly
tested. Now that we are testing it on CI with binaries compiled by LLVM
this leads to crashes with this "new" platform.

cpu/sam0_common/gpio: use gpio_irq feature

tests/periph_gpio: add interrupt enable disable command

pkg: blacklist selected `pkg`s for LLVM/clang

pkg: add libhydrogen

periph/gpio: fix doc of `periph_gpio_irq` submodule

tests/periph_gpio: make IRQ related functionality optional

drivers/tsl2561: add missing acquire∕release functions

only include irq related shell commands in case the
PERIPH_GPIO_IRQ feature is selected

sys: add missing <string.h> includes

sys: saul: add missing stddef.h include (for NULL)

boards/sensebox_samd21: Add ADC configuration

boards/sensebox_samd21: Modify ADC documentation