Integrate USB stick image generation into nrelease.

Integrate USB stick image generation into nrelease.

To build the image use:

    make img release

Notice that you might have to change some of these
variables below:

    IMGFILE ?= ${ISODIR}/dfly.img
    IMGMNT ?= /mnt   # where the image is mounted
    IMGSIZE ?= 450   # the size in MB of the image
    IMGVN ?= vn0     # the vn device used for the image
    IMGUSBDEV ?= /dev/da0 # USB device name, required for fstab

To write the image to USB disk, use:

    dd if=${ISODIR}/dfly.img of=${IMGUSBDEV} bs=1m