chroot: add option to package rootfs into directory

I'm using omap-image-builder to generate a root filesystem that is ultimately turned into a squashfs image.  The utility used to generate the filesystem image, mksquashfs, expects the filesystem source to be a directory.  I added a config option to allow the output to remain as a directory rather than getting packaged as a tarball so I can avoid the overhead of compressing and extracting during my image generation process.