When a .wks has more than one ROOTFS_DIR it's better to report
all ROOTFS_DIR that was used to create the image.
(From OE-Core rev: a8762f3be215678a6806cabe49647083f42323a8)
Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>