When bundling with CLI packager, we're getting such a message if relative classpath is mentioned (which, I'm sure, is the normal way to pass classpath):

16:27 [/tmp/testBundlerWorkDir/jars] $ "/home/dginzburg/jdk/bin/javafxpackager" -deploy -srcdir /tmp/testBundlerWorkDir/jars "-verbose" "-outdir" "/tmp/testBundlerWorkDir" "-outfile" "test" "-native" "image" "-Bclasspath=Util.jar:testFxAppWithUtil.jar" "-srcfiles" testFxAppWithUtil.jar Util.jar "-appclass" "testapp.App1"
No base JDK. Package will use system JRE.
Bundler Lunix Application Image skipped because of a configuration problem: java.lang.NullPointerException

Everything should work here: the classpath is correctly relative to current $PWD and srcdir (passed with -srcdir).
Also this example works when classpath is passed this way to ant ('<fx:bundleArgument arg="classpath" value="Util.jar:testFxAppWithUtil.jar"/>') or for bundling with Java API.

The same thing works when passing classpath in absolute paths:
16:40 [/tmp/testBundlerWorkDir/jars] $ "/home/dginzburg/jdk/bin/javafxpackager" -deploy -srcdir /tmp/testBundlerWorkDir/jars "-verbose" "-outdir" "/tmp/testBundlerWorkDir" "-outfile" "test" "-native" image "-Bclasspath=/tmp/testBundlerWorkDir/jars/Util.jar:/tmp/testBundlerWorkDir/jars/testFxAppWithUtil.jar" "-srcfiles" testFxAppWithUtil.jar Util.jar "-appclass" "testapp.App1"
No base JDK. Package will use system JRE.
Creating app bundle: /tmp/testBundlerWorkDir/bundles/App1
16:40 [/tmp/testBundlerWorkDir/jars] $ ../bundles/App1/App1
Hello World initialized!
-Dapp.preferences.id=testapp
PASS_1
JREVersion: 1.8.0_20-ea-b10

or without classpath at all:
16:42 [/tmp/testBundlerWorkDir/jars] $ "/home/dginzburg/jdk/bin/javafxpackager" -deploy -srcdir /tmp/testBundlerWorkDir/jars "-verbose" "-outdir" "/tmp/testBundlerWorkDir" "-outfile" "test" "-native" image "-srcfiles" testFxAppWithUtil.jar Util.jar "-appclass" "testapp.App1"
No base JDK. Package will use system JRE.
Creating app bundle: /tmp/testBundlerWorkDir/bundles/App1
16:42 [/tmp/testBundlerWorkDir/jars] $ ../bundles/App1/App1
Hello World initialized!
Exception in Application start method
<exception while starting because of missing Util.jar in classpath>