Implement API for GPNVM bits

[tmplt]

We can flash 0x0 if the General-Purpse Non-Volatile Memory (GPNVM) bit 1 is set: this maps the internal flash to 0x0 instead of the ROM. These bits are set via the Enhanced Embedded Flash Controller (EEFC). An API to set/get these bits may be warranted.

Originally posted by @tmplt in #6 (comment)