Release v4.2

Native border router: clearer error messages

Json library: fix a number of potential buffer overflows

Add out of tree build tests

Change build output dir structure and filenames

Apply verbosity control for mkdir, cp, srec_cat, objcopy, objdump

Reduce docker image build verbosity

Add MQTT client execution test

Only build cooja for tests that actually use it

Suitable for native run testing

Quiet npm and run all of the above without sudo

tsch: handle failure in prepare()

Improve GPIO HAL when used with 64-bit pin masks