The attached patch updates our default kernel to linux-libre 5.7.

Among other things, this will bring in-kernel WireGuard support,
multipath TCP, and USB4 (Thunderbolt).

I tested on x86_64 bare-metal and virtualized i686.

I'd like to push this as a "kernel-updates" branch and let Cuirass build
it. In the past, we always did this so that nobody had to build it
themselves. CI admins, can we do this? I don't have another way to test
for ARM platforms.

I have questions about some new features:

* Enable the compressed cache for swap pages by default? (ZSWAP_DEFAULT_ON)

Should we enable this? It seems useful and the compression algorithms
are known to be very fast. Are there any downsides?

* Enable the DMA-BUF userland memory heaps? This options creates per
heap chardevs in /dev/dma_heap/ which allows userspace to allocate
dma-bufs that can be shared between drivers. (DMABUF_HEAPS)

I don't really know what that means.

* Should USB4 (Thunderbolt) be built-in? Or a module?