- Cleans up pthread initialization, checks for ret code !=0,
instead of -1. For some platforms the first thread was not
started and therefore `pthread_join` could not succeed.
- Always specify stackaddr and stacksize to not use malloc
- Formatting and uncrustify
- Drop duplicated `cpu.c` and `cpu_conf.h`: Those are already provided by
`cpu/atmega_common`.
- The higher values for default stack size of `cpu_conf.h` in
`cpu/atmega_common` results in three tests no longer fitting the available RAM
==> Updated the Makefile.ci to skip linking of those tests for the Arduino
Leonardo
