Hi Jaroslav,
While your Yocto build will not generate a Debian-based image, you can still change the kernel configuration and device tree source for the Vybrid kernel in this build. I would recommend creating a custom layer, where in this layer, you can override the kernel defconfig to change the kernel configuration, and add a patch to the kernel source (or change the kernel source URI altogether), in order to change the device tree source. Here is some more information about creating a custom layer for your Yocto build:
Yocto Project Board Support Package Developer's Guide
As Sanchayan said, you can download and include the meta-java layer in your Yocto build. The meta-java layer can provide numerous Java utilities for your build, including the JamVM JVM.
Thanks,
Timesys Support