ext/lib : mbedTLS library modifications

Porting the mbedTLS library with the minimal Thread supported
configuration, this is a TLS/SSL library highly customizable.
This show the modifications needed to be able to compile on
Zephyr build enviroment which does not have a time.h header file,
so we avoid compilation errors.

Jira: ZEP-327
Jira: ZEP-340

Change-Id: I197ea96d33748c986c71fe5a0608225aee69c8b3
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>