libfdt: Make an internal FDT library available

This will be used by bhyve to build a device tree when booting arm64
guests.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40994

(cherry picked from commit f29af8618bf94f1e58877feb6dbef35bd8bbf56b)