mkosi: Only build system preset by default

mkosi: Only build system preset by default

Let's not build all presets by default, but only the system preset.
Also, let's only make the system preset depend on the initrd preset
if we're building a bootable disk image. This means that if we build
the system image as a cpio, uki or directory, the initrd preset will
be skipped as it's not necessary, making the build a little faster.