samples/mbedtls_sslclient: Using native IP stack

The legacy IP stack was removed from the source tree, so this changes
were made to adapt to the native IP stack, but this might not work on
the galileo platform due the ethernet driver not being tested on the
new stack on TCP, but to have the reference application ready when
it gets integrated

Jira: ZEP-1448

Change-Id: Ifa6ea0afb3bc532dfb92b09debbf6eee59e38a25
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>