ext/lib : mbedTLS Configuration modifications

Adding the mbedTLS configuration neede for Thread Network Protocol
support, this shows the modifications needed to be able to compile
on Zephyr

Jira: ZEP-327
Jira: ZEP-340

Change-Id: I80a8f44bc302905b7aa4568a40a6ca66dcaf42f8
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>