Previously, build would fail for targets containing devices with not
initramfs image (such as mpc85xx-p1010). Only generate the JSON image
info for the initramfs image when we have one to avoid breaking the
builds.
Fixes commit d3140d052964 ("build/json: generate json file for initramfs")
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 512229ce4967814a09ce202855e6b632d2e97a3c)
bb8fd58a23
