release: Pass optional VM_IMAGE_CONFIG to vm-image

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:

1. 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