Allow encrypted images using aes-128 keywrap
Allow encrypted images using aes-128 keywrap

This adds an extra method of encrypted image creation, using
AES-128-KEK. This requires a recent version of mbedtls with nist_kw
support and decreases the flash usage by 8KB from RSA-OAEP. The key file
must be provided in base64 format and it's decoded content should be a
16 byte random key.

To generate a key:

> dd if=/dev/urandom of=/dev/stdout bs=1 count=16 | base64 > aes-128-kek.b64

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt