MFC 273927:

Move the definitions of the fdt functions from a uboot header to a new fdt
header. There is nothing in the fdt spec that ties it to U-Boot.

While here sort and fix the signature of fdt_setup_fdtp.

MFC 273934:

Start to allow platforms other than U-Boot to use the FDT code in loader by
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)