nova: use smaller thread contextes in core

We need in core a lot of threads for base-nova, so reduce the thread context
area size of a thread to the actual biggest value used so far inside Core
(stack+utcb+one spare page - from 1M to 16k per thread). So we are able to
create more threads inside Core.

Issue #354
base: remove context area limitation of 256

Fixes #354
run: make bomb test ready for auto tests