Add zstd compression support
Now that 5.10 is out and supports zstd compression for the kernel and initramfs, add support for zstd to mkinitfs. The benefits have been discussed at length on lkml.
In my personal experience, the compression (mkinitfs) seems to be slower, but decompression (booting) seems to be faster. I think most people boot more than they mkinitfs.
The default is still gzip, so no change should be noticed unless someone goes looking for trouble.
To test this functionality, just add the following to mkinitfs.conf: