Currently, we create a bunch of temporary files in $(BUILD_DIR), while
assembling the filesystem images.
Move those files to their own sub-directory.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>