imagebuilder: suppress rootfs image when filesystem specified

imagebuilder: suppress rootfs image when filesystem specified

When using imagebuilder to create images, ROOTFS_FILESYSTEM may be
defined to create just the desired images, but the '*-rootfs.tar.gz'
and '*-rootfs.cpio.gz' images are being created unconditionally
for many targets.

By making generation of these images conditional on the state of
ROOTFS_FILESYSTEM, we can save significant space (measured 3-7 MB
depending on length of package list) and time.

Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>