sim: add Mbed TLS EC256 encrypted image support

Add new feature that allows testing EC256 encrypted images using the
Mbed TLS backend.

Move config-ecdsa.h to config-ec.h because definitions are very similar
between ECDSA and ECDH with Mbed TLS so resort to a single config file.

Add new feature and fix the build; add proper Mbed TLS memory
initialization when enc-ec256-mbedtls is used.

Signed-off-by: Fabio Utzig <utzig@apache.org>