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 d3140d0529 ("build/json: generate json file for initramfs")
Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: maurerr <mariusd84@gmail.com>