bhnd(4): Implement support for directly converting a bhnd_nvram_val to a new type.

[landonf]

Summary

This adds support for bhnd_nvram_val_convert_init() and bhnd_nvram_val_convert_new(),
which may be used to perform value format-aware encoding of an NVRAM value to a new
target format/type.

This will be used to simplify converting to/from serialized format-specific NVRAM value
representations to common NVRAM value representations.