[SAMR21] change stack allocation to use all free RAM (#2334)
[SAMR21] change stack allocation to use all free RAM (#2334)

Initially fixed size 8k was allocated for stack.
As result there was almost no free RAM. This
change creates stack on all free RAM.

Signed-off-by: Oleksandr Grytsov <a4284420daa50c9a032f36f0da83444a864c07ab@gmail.com>
openthread/openthread,chshu/openthread,georgecpr/openthread,erja-gp/openthread,openthread/openthread,gandreello/openthread,pvanhorn/openthread,chshu/openthread,mszczodrak/openthread,gandreello/openthread,LeZhang2016/openthread,abtink/openthread,mszczodrak/openthread,abtink/openthread,LeZhang2016/openthread,chshu/openthread,lanyuwen/openthread,LeZhang2016/openthread,mszczodrak/openthread,erja-gp/openthread,georgecpr/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,chshu/openthread,erja-gp/openthread,srickardti/openthread,jwhui/openthread,gandreello/openthread,pvanhorn/openthread,georgecpr/openthread,turon/openthread,erja-gp/openthread,turon/openthread,abtink/openthread,jwhui/openthread,xiaom-GitHub/openthread,librasungirl/openthread,erja-gp/openthread,chshu/openthread,pvanhorn/openthread,bukepo/openthread,mszczodrak/openthread,mszczodrak/openthread,librasungirl/openthread,lanyuwen/openthread,pvanhorn/openthread,erja-gp/openthread,mszczodrak/openthread,gandreello/openthread,xiaom-GitHub/openthread,srickardti/openthread,bukepo/openthread,turon/openthread,jwhui/openthread,librasungirl/openthread,LeZhang2016/openthread,lanyuwen/openthread,openthread/openthread,bukepo/openthread,pvanhorn/openthread,abtink/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,openthread/openthread,librasungirl/openthread,turon/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,xiaom-GitHub/openthread,lanyuwen/openthread,jwhui/openthread,georgecpr/openthread,srickardti/openthread,georgecpr/openthread,pvanhorn/openthread,gandreello/openthread,georgecpr/openthread,gandreello/openthread,bukepo/openthread,LeZhang2016/openthread,lanyuwen/openthread