Drop dependency on /etc/fstab for /boot

[cgwalters]

Splitting this sub-thread from #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 (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...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.