fix(sptool): add dependency to SP image

In the generated sp_gen.mk, add a dependency to the image described in
the sp_layout.json file to make sure that the pkg file is re-generated
if the SP image is updated.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Change-Id: Id936f907d6baa6b0627c4bb9608323e5157c7a9b
1 file changed