makefile condition

src/freedombone-image-makefile

 ARCHIVE = $(IMAGE).xz
 SIGNATURE = $(ARCHIVE).sig
 OWNER = 1000
-if [ ! "$CONTINUOUS_INTEGRATION" ]; then
+ifeq ("$CONTINUOUS_INTEGRATION", "")
+	XZ =
+	SIGN =
+else
     XZ = xz --no-warn --verbose --keep --threads=0 -3
     SIGN = -gpg --output $(SIGNATURE) --detach-sig $(ARCHIVE)
-fi
+endif
 
 # settings for `make test`
 TEST_SSH_PORT = 2222