software/app: Make it CPU/GPU compatible (if GPU support is required).

[enjoy-digital]

Most of the Gateware development is done on a machine without a GPU and with regular LitePCIe driver. If GPU support is wanted for this software/app, we should also be able to compile it/use it on a machine without a GPU, without NVIDIA tools installed and with regular LitePCIe driver.

For now to continue my tests, the version I was using for the development has been reverted with https://github.com/enjoy-digital/xtrx_julia/commit/fc1860e2967e7463334b3b41b95423dea9eed9b3.