Merge branch 'develop' into contrib/tsch-max-payload

.travis.yml

arch/cpu/cc26x0-cc13x0/dev/contiki-watchdog.c

arch/cpu/cc26x0-cc13x0/lpm.c

arch/platform/cooja/Makefile.cooja

arch/platform/cooja/contiki-conf.h

arch/platform/cooja/dev/buttons.c

arch/platform/cooja/dev/gpio-hal-arch.c

arch/platform/cooja/dev/gpio-hal-arch.h

arch/platform/cooja/dev/leds-arch.c

arch/platform/cooja/platform.c

examples/dev/button-hal/Makefile

examples/dev/gpio-hal/Makefile

examples/dev/gpio-hal/cooja/pins.c

examples/hello-world/hello-world.nrf52840dk

examples/rpl-border-router/border-router.nrf52840dk

examples/snmp-server/Makefile

examples/snmp-server/project-conf.h

examples/snmp-server/resources/snmp-SNMP-MIB-2-System.c

examples/snmp-server/snmp-server.c

os/contiki-main.c