boot-directdisk.bbclass: use rootfs UUID by default
This changes the default SYSLINUX_ROOTFS such that the rootfs is no
longer expected under a fixed device path. Instead, the UUID is used
to find it. This makes the resulting .hdddirect (and thus also the
vdi/vdmk/qcow2 images derived from that) more flexible.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>