fstab: always mount sysroot rw

EOS-2.1.0 shipped with /sysroot mounted here as read-only, and "ro" in
the kernel boot args, and a static /etc/machine-id in ostree for all
installations.

To correct the machine-id situation, machine-id has been removed from
ostree. So when upgrading from EOS-2.1.0, /sysroot must be mounted rw
so that machine-id can be generated early during first boot into the
upgraded OS.

We don't have any mechanism to fix up the boot args during the upgrade;
we can't fix this in ostree because the upgrade is driven by the ostree
version already shipped in EOS-2.1.0.

Instead, this change makes the ro/rw parameters be ignored at the
initramfs
level, now we always mount sysroot rw.

https://github.com/endlessm/eos-shell/issues/3171
hostnamed: prefer DMI information to ACPI for chassis type

At least for the devices we care about, this seems to be more
reliable...