mbedtls: new package

Add the mbed TLS library, formerly known as PolarSSL.
The 2.0+ version can live side-by-side with polarssl since it was
renamed, hence can be added with no conflict.

Use the Apache-2.0 license variant since it's somewhat more flexible,
the codebase is the same as the GPLv2 variant, just copyright
headers/license file changed.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>