Use image-artifact-names

OE-core has moved IMAGE_ variables to a separate class, and they are not
found without image-artifact-names class.
Signed-off-by: Stefano Babic <sbabic@denx.de>
Reported-by: Parthiban Nallathambi <parthiban@linumiz.com>
Tested-by: Parthiban Nallathambi <parthiban@linumiz.com>

classes/swupdate.bbclass

@@ -32,6 +32,7 @@
 #   image file with exactly the name as specified in SWUPDATE_IMAGES is searched for.
 
 inherit swupdate-common.bbclass
+inherit image-artifact-names
 
 S = "${WORKDIR}/${PN}"