Allow the java part to be built locally

[llewelld]

In theory, it should be possible to build the Java part inside an sfdk target, rather than on a remote device.

Here's a proof of concept to show how this might work, if it worked: #1

In practice QEMU gets upset about the amount of memory the JVM requires. Restricting the JVM heap to 1Gb allows things to progress (albeit very slowly) but eventually the build fails at the build-native stage when the JVM runs out of heap.

Newer versions of QEMU don't seem to be limited in this way, so it's possible a future sfdk update might allows this to go through (in which case the restrictions on the JVM heap size would need to be removed).