tests/arduino: add automatic test for arduino module

pkg/openthread: rework of FTD and MTD support

lwip: Bump version to v2.1.0

drivers/adxl345: fix ADXL345 driver

drivers/periph_eeprom: refactor implementation and test application

core/bitarithm: change types to `uintXX_t`

ESP32 port

drivers/netdev_ieee802154: cleanup NETOPT_MAX_PACKET_SIZE

llvm.inc.mk: completely replace detection of GCC includes

net/nanocoap: fix response type

This completely removes the hardcoded/os specific path setting for includes.
It directly queries gcc include directories.

It takes what was already done by `makefiles/libc/llvm.inc.mk`.

I replaced the `GCC_MULTI_DIR` handling by givining `CFLAGS_CPU` when
searching for include directories.
`CFLAGS` cannot be used as it will crash when `-target $(CPU_ARCH)` is
added.
It currently requires using deferred variables as `CFLAGS_CPU` can be
overwritten later in the build process.

boards: add Zolertia Firefly board

sys/arduino: add arduino time functions micros() and delayMicroseconds()

Patch cherry-picked into upstream as [a47d30f].

[a47d30f]: http://git.savannah.nongnu.org/cgit/lwip.git/commit/?id=a47d30ffc7d36a79307486ca5e9a48f5bf89c7b6

See release [blog] post for details.

[blog]: https://savannah.nongnu.org/forum/forum.php?forum_id=9248

lwIP uses a lot of macro "magic" (including dereferencing then
pointerizing pointers again) that confuses the static code checker of
most compilers.