tls: integration of atecc608b to perform authentication

[joelguittet]

The purpose of this ticket is to provide TLS authentication using a secure element. An example should be shown.

For this purpose the current tls implementation should be reworked to keep private key inside the tls module. API with the mender-client, mender-api and mender-storage modules will move.

The cryptoauthlib from Microchip will be used to interface the atecc608b secure element.

Tasks includes:

• Management of private key inside the tls module
• Selection of the tls implementation through CMake and KConfig files (choices should be mbedtls or cryptoaauthlib for atecc608b)
• Creation of a new specific example to demonstrate the usage of the secure element
• Update of the documentation

References:

• https://github.com/MicrochipTech/cryptoauthlib
• https://github.com/espressif/esp-cryptoauthlib (provide an easy way to perform atecc608b provisioning)