buildworld has mandatory dependency on optional executable.

I've just discovered that "make buildworld" has a mandatory dependency
on kbdcontrol (see
https://svnweb.freebsd.org/base/head/Makefile.inc1?annotate=354138#l2207 )
but, if WITHOUT_LEGACY_CONSOLE is defined then kbdcontrol isn't built
(https://svnweb.freebsd.org/base/head/usr.sbin/Makefile?annotate=352949#l162 )
and the installed version will be deleted by "make delete-old":
https://svnweb.freebsd.org/base/head/tools/build/mk/OptionalObsoleteFiles.inc?annotate=353358#l4520

This seems undesirable...

The "make buildworld" failure doesn't make the cause obvious - it just
reports "*** Error code 1" in bootstrap-tools.  Having trace the failure,
I now see ".ERROR_TARGET='_bootstrap-tools-link-kbdcontrol'" but that was
only obvious in hindsight.

-- 
Peter Jeremy