hostap: Reserve the heap

By default (-1) libc uses all the available heap, this is good to
efficiently use the RAM, but for scenarios which there isn't enough RAM
the build will be success, but we see runtime failures.

Reserved the necessary RAM based on tests ahead to catch such scenarios,
of course this would mean that we might be no using full RAM in case
more is left, but that's never the case the with Wi-Fi :).

And this also adds the constant evaluation and fine tuneing like we do
for kernel heap and other parameters.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>