If FP_DEVICE_EMULATION is set, then switch to using predictable EC
ephemeral key and random numbers. This should allow recording and
replaying real device interactions using umockdev.
This adds a base class for SDCP devices. Not all functionality has been
fully tested, in particular the code to verify the model certificate is
most likely broken or incomplete. One problem there is that there is no
code to find the root CA to trust.
See: #257