bhnd(4): Add support for exporting all (or a subtree) of NVRAM
properties backed by an NVRAM store.

This will be used to support:

• Serializing the current NVRAM state for writing back to flash.
• Exporting subsidiary device paths for serialization and upload to fullmac chipsets.

Additionally, this includes an improvement to BCM-RAW format detection
to avoid matching on BCM-TEXT NVRAM data.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8761