core/msg: revert part of #1285

use m->sender_pid instead of sched_active_pid again
undoes the optimization intended by #1285
fixes uint/int warning
core/msg: set m->sender_pid before it is used

fixes `msg_send_to_self` condition
SQUASHME: do it right
make: move exported make vars to one place
make: don't use export on example makefiles
Merge pull request #1109 from Kijewski/make-exports

make: no "unfriedly" `exports` in the application's Makefile
Merge pull request #1387 from LudwigOrtmann/msg_fixup

core/msg: clean up after #1285
core: align stack on a 32bit boundary

Fixes #1267.
Add container_of macro
core: rename tcb_t in thread_t
 core: split TCB from stack

 This eliminates problems like #1267 and is architectural easier to
 understand. I think it will help debugging, too, because you can much
 easier introspect threads at runtime.