Add a section on the testing procedure for a pull request.
It should help reviewing pull requests as they often not describe which
application raises the error they are fixing or any way of verifying
that a feature is correctly implemented.

drivers/adxl345: fixes & cleanup

posix/pthread_rwlock: fix unsigned variable comparison

cpu/nrf52: add PWM implementation

tests/driver_isl29125: fix of compilation error

drivers/periph_common/eeprom: move EEPROM definitions to periph_cpu.

murdock: also compile with LLVM/clang

Fmt: add fmt hex byte

There are two major reasons for this:

1. clang picks up different errors sometimes than GCC.
2. OSX support is hardened as it is usually the toolchain used there.

at86rf2xx: correct rssi

dist/tools: add lazysponge tool

Remove file management from `genconfigheader` script and use `lazysponge` in
Makefile.include.

Use --verbose option when in non QUIET building mode.

Write stdin to <outfile> if it is different from the previous content.

If data provided in stdin is the same as the data that was in <outfile>, it is
not modified so `last modification date` is not changed.

boards: Extended/fixed SPI config for STM32 Nucleo-64 boards

tests/lwip_sock_*: fix compilation error

cpu/stm32_common: add PUF SRAM feature to makefile

boards/pba-d-01-kw2x: use OpenOCD-supplied config

at86rf2xx: Move flags from netdev to radio

gnrc_sixlowpan_iphc: cleanup NHC encoding

boards: Harmonized UART config of Nucleo-64 boards

boards/stm32f103c8: update flash flags for DFU

nimble: deactivate -Wunused-function with LLVM/clang

When building `nimble` with LLVM/clang the function
`ble_ll_adv_active_chanset_is_sec()` in `ble_ll_adv.c` is reported to
be unused, it is used within an assertions and `ifdef`s within that
file though. So this just disables that warning.

make: provide support for listing supported and blacklisting toolchains

Assuming `TOOLCHAIN_SUPPORTED` is provided by the board and
`TOOLCHAIN_BLACKLIST` by a module or `pkg`, this outputs the fact that
a toolchain is not supported or blacklisted in a similar manner as
(un-)supported features and boards are.