test: custom_handler.sh: adapt to new target slot selection
test: custom_handler.sh: adapt to new target slot selection

With the image-independent target slot selection, not all slots listed
in RAUC_TARGET_SLOTS must also have an image assigned to.

Thus we additionally have to check if we have a valid installation image
for this slot present before triggering an installation.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc