arm-gcc version from ubuntu bionic repository is not supported in RIOT.
Both when building with `gnu` and `llvm`.

    arm-none-eabi-gcc --version
    arm-none-eabi-gcc (15:6.3.1+svn253039-1build1) 6.3.1 20170620

So detect this version and print an error if found.

The check is done on the building machine and not on the host when building in
docker.

The error can be disabled when building with WERROR=0.

 gnrc_sixloenc: introduce pseudo-module to send 6Lo frames over Ethernet

gnrc: make titles of NETAPI command documentation more specific

tests/xtimer_drift: increase priority of worker

gnrc_ipv6: make gnrc_netapi documentation more detailed

drivers/ads101x: eliminate casts

cpu/esp32: Fixes the maximum packet size of 255 bytes in the esp_eth netdev driver of ESP32 mcu.

Fixes the maximum packet size of 255 bytes in the esp_eth netdev driver of ESP32 mcu.

fixup! cpu/esp32: fixes esp_net maximum packet size

cortexm_common: add FPU support for cortex-m4f and cortex-m7

pkg: gecko_sdk: bump version + vendor headers

A helper variable is added, and now all type casts are not needed anymore.

 netopt: provide option to (de-)activate 6Lo

tests/periph_uart: fix baudrate truncation

net/nanocoap: allow user to configure macros

net/coap: allow user to configure message retry macros

Since some boards an int is 16 bits the atoi truncates values.
This commit using a long instead of an int.

net/gcoap: add config macros to config doc group

tests/driver_ds1307: Blacklist iotlab boards

This states iotlab boards shouldn't run these tests.
This is because the i2c address of L3G4200 is the same ad the ds1307.
This causes an error that thinks the test is valid when it is not.

boards/common: change openocd config file for stm32l1x

cpu/nrf5x: move periph_gpio/periph_gpio_irq at an even more common level

tests/sx127x: set b-l072z-lrwan1 as default board

loramac: change rxdelay1 to millisec, was seconds

net/nanocoap: remove obsolete macro

 gnrc_ipv6_ext_rh: add ICMPv6 error message sending