Move inclusion of src.opts.mk later.

src.opts.mk includes bsd.own.mk. This in turn defines CTFCONVERT_CMD
depending on the MK_CTF value. We then set MK_CTF to no, which has no
real effect. The solution is to set all the MK_foo values before
including src.opts.mk.

This should stop the cdboot binary from exploding in size for releases
built WITH_CTF=yes in src.conf.

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