Wrap CTFCONVERT invocations in ${defined(CTFCONVERT):? ... :}
so it performs the test at the time the rule is invoked.  We can't
use a ".if defined" test because that would be tested at the time the
sys.mk file is parsed, which may be before CTFCONVERT has been set by
bsd.own.mk.

Remove the now-unnecessary assignment "CTFCONVERT ?= : ctfconvert",
and the similar assignment to CTFMERGE.  Now the build logs are not
cluttered by many unnecessary invocations of the ": ctfconvert ..."

(apb)