build-vm: 07-kernel: all initrd modules are optional

The problem at hand is that different kernels can have
varying module sets, and it makes sense to put four of
those at once sometimes; so avoid silly build breakage.

features.in/build-vm/image-scripts.d/07-kernel

@@ -15,7 +15,7 @@ cd /boot
 for KVER in $kver; do
 	make-initrd -N -k "$KVER" AUTODETECT= \
 	  FEATURES+="$GLOBAL_VM_INITRDFEATURES" \
-	  MODULES_ADD+="$GLOBAL_VM_INITRDMODULES" \
+	  MODULES_TRY_ADD+="$GLOBAL_VM_INITRDMODULES" \
 	  || { echo "** Error: make-initrd failed" >&2; exit 1; }
 done