build-appliance: make the inclusion of downloaded sources optional

build-appliance: make the inclusion of downloaded sources optional

Including the entirety of DL_DIR in the generated build appliance
image adds a significant amount of space and makes the build
appliance image more awkward to distribute. Add a configuration
option to make the inclusion of sources option and default to
disabling this functionality.

(From OE-Core rev: 7975325e3d8dee159752ae2304b0c00361bfd2a7)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>