fdt: further consolidate DTB building and revise manpage

FDT_DTS_FILE was built separately with a rule in sys/conf/files and
recreated the rules we used in dtb.mk. Now that we have other infrastructure
to build a DTB along with the kernel, fold FDT_DTS_FILE into that since it
doesn't have any special requirements.

fdt(4) never got revised to mention the DTS/DTSO make options, so do that
now.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19736