This adds support for:
- Serializing an bhnd_nvram_plist (as exported from bhnd_nvram_store, etc) to
an arbitrary NVRAM data format.
- Generating a serialized representation of the current NVRAM store's state
suitable for writing back to flash, or re-encoding for upload to a
FullMAC device.
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8762
378d13bcd3