MFC r346307, r346618: Further DTB building consolidation/documentation

r346307:
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.

r346618:
fdt: stop installing FDT_DTS_FILE

r346307 inadvertently started installing FDT_DTS_FILE along with the kernel.
While this isn't necessarily bad, it was not intended or discussed and it
actively breaks some current setups that don't anticipate any .dtb being
installed when it's using static fdt. This change could be reconsidered down
the line, but it needs to be done with prior discussion.

Fix it by pushing FDT_DTS_FILE build down into the raw dtb.build.mk bits.
This technically allows modules building DTS to accidentally specify an
FDT_DTS_FILE that gets built but isn't otherwise useful (since it's not
installed), but I suspect this isn't a big deal and would get caught with
any kind of testing -- and perhaps this might end up useful in some other
way, for example by some module wanting to embed fdt in some other way than
our current/normal mechanism.