Currently the flag "-P ${PORT}" is added to avrdude regardless of the programmer
used. But this flag should only be set for programmers that operate over a
serial port - e.g. like the various Arduino bootloaders. This commit changes
the behaviour so that the "-P flag" is only set for only of the default
programmers of the various AVR boards supported by RIOT. This allows to use
ICSP programmers (e.g. like the usbtiny) like this:

    make BOARD=arduino-uno PROGRAMMER=usbtiny

tools/serial: add support for socat as riot terminal

makefiles/serial: fix indent

sys/random: use periph/hwrng as seed if available

pkt/emb6: fix typo in hwrng call

ds323x: Minimal driver for DS323x extremely accurate RTC

This driver currently only supports the SPI connected DS3234, and only
for initializing 1 Hz square wave output on the SQW pin, nothing else.

gnrc_tcp: change and verify addrs in use

drivers/sx127x: fix dead links

sys/event: make event queue independent from thread

URLs of the datasheets have been changed.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

drivers: Add AD7746 capacitance sensor driver

gnrc_sixlowpan_frag: remove redundant PID member

It can't get bigger, so it doesn't make sense to waste another 2 byte
on it.

The interface ID is actually also provided in `pkt` so providing it in
the `gnrc_sixlowpan_msg_frag_t` struct is redundant.

drivers/hts211: fix CTRL_REG2_OS_EN

fixes #11013

Release 2019.01 notes

net/gnrc/pktbuf.h: add note in gnrc_pktbuf_mark

Co-authored-by: Emmanuel Baccelli <emmanuel.baccelli@inria.fr>
Co-authored-by: Gaëtan Harter <gaetan.harter@fu-berlin.de>
Co-authored-by: Martine Lenders <m.lenders@fu-berlin.de>
Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
Co-authored-by: Peter Kietzmann <peter.kietzmann@haw-hamburg.de>

gnrc_ipv6_nib: fix preparation of delayed NA

tests/periph_eeprom: use EEPROM_CLEAR_BYTE

gnrc_ipv6_nib/arsm: ensure proper int width in backoff calculation

dist/tools: Add script for backporting PR's

This was primarily done so the backoff can be tested

Otherwise, the result might flow over on 8/16-bit platforms