camera-assistant: move files to files subdirectory

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
layer.conf: use ??= to set variables

If these variables are set with = or ?= then the ?= setting in
conf/distro/include/preferred-opie-git-versions.inc will not be used.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>