Although the request flavour to install is oem-20.04, it really is called just oem on disk. Override the flavour name from oem-20.04 to oem when renaming built artefacts. This also means that ubuntu-cdimage needs to simply download 'oem' vmlinuz+initrd pairs, not 'oem-20.04'.

debian/changelog

@@ -1,3 +1,12 @@
+livecd-rootfs (2.640) UNRELEASED; urgency=medium
+
+  * Although the request flavour to install is oem-20.04, it really is
+    called just oem on disk. Override the flavour name from oem-20.04 to
+    oem when renaming built artefacts. This also means that ubuntu-cdimage
+    needs to simply download 'oem' vmlinuz+initrd pairs, not 'oem-20.04'.
+
+ -- Dimitri John Ledkov <xnox@ubuntu.com>  Thu, 30 Jan 2020 11:10:37 +0000
+
 livecd-rootfs (2.639) focal; urgency=medium
 
   * On s390x subiquity:

live-build/auto/build

@@ -819,10 +819,14 @@ for FLAVOUR in $LB_LINUX_FLAVOURS; do
 	if [ -z "$LB_LINUX_FLAVOURS" ] || [ "$LB_LINUX_FLAVOURS" = "none" ]; then
 		continue
 	fi
-	if [ "$FLAVOUR" = "virtual" ]; then
-		# The virtual kernel is named generic in /boot
-		FLAVOUR="generic"
-	fi
+	case $FLAVOUR in
+		virtual)
+			FLAVOUR="generic"
+			;;
+		oem-*)
+			FLAVOUR="oem"
+			;;
+	esac
 	KVERS="$( (cd "binary/$INITFS"; ls vmlinu?-* 2>/dev/null || true) | (fgrep -v .efi || true) | sed -n "s/^vmlinu.-\\([^-]*-[^-]*-$FLAVOUR\\)$/\\1/p" )"
 	if [ -z "$KVERS" ]; then
 		if [ -e "binary/$INITFS/vmlinuz" ]; then