Firmware always rebuilding

[zrecore]

@mithro says that incremental building of the firmware is broken again and should be fixed.

[TomKeddie]

@mithro can I take this one?

[mithro]

@TomKeddie Sure! The most likely cause is the modtime on the generate files changing even though their contents are not changing. I previously solved this like https://github.com/enjoy-digital/litex/commit/8179a9ea2e1a0233c5b9f9caca68a5463f68eb61