ext: lib: mbedtls: Enable and set heap size at device startup

As the mbedtls heap is global for the whole device, enable it
during device startup if configured so. The heap size can be
set in config file. There is no default value for the heap as
that depends very much on application needs.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>