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>
diff --git a/sim/mcuboot-sys/Cargo.toml b/sim/mcuboot-sys/Cargo.toml
index 19114f9..ac203f1 100644
--- a/sim/mcuboot-sys/Cargo.toml
+++ b/sim/mcuboot-sys/Cargo.toml
@@ -44,6 +44,9 @@
 # Encrypt image in the secondary slot using ECIES-P256
 enc-ec256 = []
 
+# Encrypt image in the secondary slot using ECIES-P256 using Mbed TLS
+enc-ec256-mbedtls = []
+
 # Encrypt image in the secondary slot using ECIES-X25519
 enc-x25519 = []