Support for multiple initrds

[arcriley]

Added support for several initrds which are provided to the kernel concatenated into one buffer. Most tools that build initrds (eg Dracut) do this for you but there are some situations where this can be extremely useful.

A single string is still supported this will still work
chainload.linux-initrd = 'path/to/initrd'

but now an array of initrds are also supported which as a good feel with other kernel arrays in sprout.toml;
chainload.linux-initrd = ['path/to/initrd1', 'path/to/initrd2']
chainload.options = ["root=rootfs", "ro", "init=/usr/lib/systemd/systemd"]

This still supports undefined initrd and empty_is_none* functions were removed as now obsolete.

[azenla]

Will review this weekend!

[arcriley]

Will review this weekend!

I am mostly looking for feedback on the code styling before I put too many hours into the next steps.

The purpose of splitting initrd is to use signed hashes. It would be trivial to hide malware in a monolithic initrd.

However this was just a low hanging fruit. I am mostly looking for an experienced Rust developer to review this before I put too much additional time into boot selection or other more complex PRs.

If you have other repositories that are higher priority please point me at them, I am job hunting and looking to contribute to Rust projects refresh my familiarity. You can use me for free code work until I am locked into another "no compete" deal.