pkg/39715: bootstrap-mk-files/files/bsd.files.mk nuking FILES

[David Brownlee <abs%absd.org@localhost>]

pkg/39715 notes that bsd.files.mk unconditionally nukes FILES at the
end to allow multiple inclusions, and proposes switching to a less
antisocial:

.if !defined(_BSD_FILES_MK)
_BSD_FILES_MK:=       1
[...]
.endif

paradigm instead. Can anyone see any issues with this change?

Thanks