Update the salt script + releases files

[palainp]

This PR wants to solve #200 and emphasises the separation of sha256 for released versions and for the current version of head. This way we can push commits on top of head, continue to ensure reproducible builds, and also keep something to compare the shasum of the released version. The PR process will need updating qubes-firewall.sha256, and the release process need to copy this file into qubes-firewall-release.sha256. WDYT?

I'm unsure whether the salt script has been updated correctly and I'll have to wait for the next version to fix any issues.

[hannesm]

I think the approach is fine.

[palainp]

I still have an issue the first time I run the script, because the directory for the kernel is not yet created. So far I'm unsure if I have a bad usage of salt or if just need to create the directory if test -e fails on the release version check EDIT: fixed, we now create the directory and always touch the version file:

$ sudo qubesctl --show-output state.apply SaltScript...
[ERROR   ] Command 'test' failed with return code: 1
[ERROR   ] stdout: touch: cannot touch '/var/lib/qubes/vm-kernels/mirage-firewall/version.txt': No such file or directory
cat: /var/lib/qubes/vm-kernels/mirage-firewall/version.txt: No such file or directory
[ERROR   ] retcode: 1
[ERROR   ] Command 'test' failed with return code: 1
[ERROR   ] output: touch: cannot touch '/var/lib/qubes/vm-kernels/mirage-firewall/version.txt': No such file or directory
cat: /var/lib/qubes/vm-kernels/mirage-firewall/version.txt: No such file or directory