Refactor LMIC to support secure element

[terrillmoore]

MCCI is doing an integration of the LMIC with an external secure element. For this work, the LMIC needs to be refactored to move crypto out of the main body of LMIC (so that the LMIC doesn't have to hold the keys). This will be a major change, but worthwhile for security.

This issue is being created so that MCCI can create a branch to track development of this feature.