zynq: various fixes and board support

[fsagbuya]

Description

This PR aims to unify all zynq image related patches.

List of patches from this PR:

1. Add support for u-boot wrapped kernel and initrd (Xilinx recommended).
2. Use the zc706 zynq variant.
3. Add hostname.
4. Enable login.
5. Remove the qemu script from the build, so users can freely modify this somewhere like a flake. This can make the system more flexible, while keeping only the minimal components accessible.
6. Add boot from sd image support.

Combine the changes from: #26 and #27.

[fsagbuya]

Hi @cleverca22, could you take a look at these PRs when you get a chance? Thanks!

• zynq: various fixes and board support #28
• nixpkgs update fixes #29
• enable host key generation on first boot #30
• Configure shutdown instructions in runit stage 3 #32
• Enable FPGA programming support for Zynq-7000 #33