Make kairos-sysext be generic enough to include on all flavors

[Itxaka]

It will not install the systemd-sysext package if the binary is not there

[Itxaka]

Test ongonig for this: kairos-io/kairos#1904

[Itxaka]

Expected: On systems that do not support systemd-sysext becuase they are older, its not installed, on systems that support it, it is.

Outcome of the test:

on ubuntu-20-lts (no sysext available): +base-image | dracut: dracut module 'kairos-sysext' will not be installed, because command 'systemd-sysext' could not be found!
on ubuntu-22-lts (no sysext available): systemd-sysext module was NOT added to initramfs. Silent skip due to the comments in the code, mainly it has the binary but not the dracut module, so we add extra checks and fail in a different place.

on opensuse-tumbleweed (sysext available), systemd-sysext module was added to initramfs
on opensuse-leap (sysext available), systemd-sysext module was added to initramfs
on ubuntu (sysext available), systemd-sysext module was added to initramfs
on almalinux (sysext available), systemd-sysext module was added to initramfs
on fedora (sysext available), systemd-sysext module was added to initramfs
on debian (sysext available), systemd-sysext module was added to initramfs
on rockylinux (sysext available), systemd-sysext module was added to initramfs

So as a side effect of this generic approach of adding if its there, we GAIN sysext support in all newer distros :D

[Itxaka]

This indicates to dracut to always include the module

[Itxaka]

this is what brings the systemd-sysext module as dependent, so it includes on systems that have botht he binary and the dracut module available, otherwise this module depends on nothing and its basically an empty thing that does nothing.

[mauromorales]

very nice