release: Pass optional VM_IMAGE_CONFIG to vm-imageHEADmain

`make vm-image` calls mk-vmimage.sh, which supports
`-c CONFFILE`. This file gets sourced before building the image.

One example of how to use it is to define
vm_extra_filter_base_packages() to filter the list of packages
installed into the VM image:

    # vm-nodbg32.conf
    vm_extra_filter_base_packages() {
        grep -v -E '(-dbg|lib32)'
    }

    $ make VM_IMAGE_CONFIG=path/to/vm-nodbg32.conf \
        VMFORMATS=raw \
        -DWITH_VMIMAGES \
        vm-image

Signed-off-by: Pat Maddox <pat@patmaddox.com>
Reviewed by:	cperciva
Closes:		https://github.com/freebsd/freebsd-src/pull/1993

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