dfmem unique ID

This pull request is for the small code addition to the dfmem module that will read the factory unique ID from the flash memory. The register is read on board start-up, and a getter is provided.

Note that some other changes from other files have gotten included here, where a new XC16-style include for the chip specific header file is also done here. I'm unsure how to partition those changes from just the dfmem.c/h new code.