moving U-Boot specific code from libfdt.a to a new libuboot_fdt.a. This
needs to be a new library for linking to work correctly.
Differential Revision: https://reviews.freebsd.org/D1054
Reviewed by: ian, rpaulo (earlier version)
MFC after: 1 week
o This is disabled by default for now, and can be enabled using WITH_FDT at
build time.
o Tested with ARM and PowerPC.
Reviewed by: imp
Sponsored by: The FreeBSD Foundation