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()

It is used by Makefile.base so should be exported globally even if only
defined by `llvm.inc.mk` for the moment.

drivers/vcnl4010: initial implementation

pkg/semtech-loramac: use internal eeprom to store lorawan config (deveui, appeui, etc)

Add note about save and erase loramac subcommands

cpu, periph_gpio: mark closing #endif for MODULE_PERIPH_GPIO_IRQ

net/nanocoap: add module documentation

Adding additional information about the default uart configuration to the bluepill docs

coap: Replace `COAP_CT_` with `COAP_FORMAT_`

boards/remote-revb: Update broken link in readme

Changes the default port on LINUX systems to /dev/ttyUSB1. /dev/ttyUSB0 is the JTAG interface for OCD.

COAP_CT_ style defines for the content types are deprecated in favour of
COAP_FORMAT_ style defines. COAP_FORMAT_ is expanded to include any
missing content type that was available with COAP_CT_.

Replaces all occurences of COAP_CT_.* with COAP_FORMAT_.*

make: fix info-objsize column name position

pkg: add libb2 (BLAKE2)

The problem seemed to be a pipelining problem of write and read instructions when swapping the context. An isync instruction when returning from a context switch solves the potential pipelining problem.

nanocoap: add server-side block2 support