bhnd(4): support direct conversion of bhnd_nvram_val

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 external
representations.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8757