Drop dependency on `/etc/fstab` for `/boot`

Splitting this sub-thread from https://github.com/containers/bootc/issues/357

Today `bootc install to-filesystem` always writes an `/etc/fstab` entry for `/boot`. However, I think the OS container should be in control of the semantics more here (things like mount options, filesystem type).

One strawman here is that we could look for a `/usr/lib/systemd/system/boot.mount` unit, and template it (adding the final uuid) into `/etc/systemd/system`.

The other option is for `bootc install` to switch to generating a `boot=` kernel argument...one thing is this sadly is [required for FIPS compliant mode](https://github.com/dracutdevs/dracut/blob/4980bad34775da715a2639b736cba5e65a8a2604/modules.d/01fips/fips.sh#L37) (to compare the booted kernel in the initramfs), and so we might as well do it unconditionally. We'd then probably argue to extend systemd-fstab-generator for this (or carry an equivalent generator elsewhere).

Yet another model...since the bootloader had to parse the `/boot` filesystem anyways, get that information passed to us. I [commented earlier on this](https://github.com/coreos/fedora-coreos-tracker/issues/465#issuecomment-615286296)...this seems like the most elegant, but also hardest as in the general case we'd need to get a coordinated change across systemd-boot, grub, zipl etc.