unittests: do not use auto_init
core: clist is a cyclic list
core: clist without explicit thread

Right now the core component `clist` is a generic cyclic doubly-linked list.
In the core it is used in `tcb_t::rq_entry`.
Further it is used `net_if.c`.

This commit removes the member `clist_node_t::data` which stored the
pointer to the `tcb_t` instance of which the clist is already a member.
The needless member added `sizeof (int)` bytes to every instance of
`tcb_t`.

In `net_if.c` the clist was used in a type-punned way, so that the
change won't affect it.
Merge pull request #1013 from Kijewski/clist_wo_thread

 core: clist without explicit thread
tests: Fix tests that were defect

* I used pexpect instead of when expect script was not working expect, since
  TCL confuses me
* I deleted the thread_exit test since it was completely invalid
Merge pull request #1596 from authmillenon/fix-expect

tests: Fix tests that were defect
Merge pull request #1594 from authmillenon/unittests-no-auto-init

unittests: do not use auto_init
boards:make: hardcode goodfet.bsl path in Makefile.include
goodfet: hardcode python version to 2.7
Merge pull request #1573 from thomaseichinger/hardcode_goodfet

boards:make: hardcode goodfet.bsl path in Makefile.include
cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist board arduino-mega2560 for tests/examples that don't build
sys: net_help: inet_pton: include sys/types.h conditionally for atmega2560

    Without this test/unittests fails to build due to a missing declaration of
    ssize_t
cpu: atmega2560: add conditional thread_yield to timer implementation
pu: atmega2560: add conditional thread_yield to uart implementation
cpu: atmega2560: include modules lib for ringbuffer in uart implementation
board: arduino-mega2560: switch to interrupt based uart implementation for rx