Grub serial console support

The standard grub2 configuration is completely ignored as bootupd is used for bootc systems, so this will not work at all:

RUN cat > /etc/default/grub <<'EOF'
GRUB_TERMINAL="console serial"
GRUB_SERIAL_COMMAND="serial --speed=115200"
EOF

Bootc uses static grub config, luckily it include:

if [ -f $prefix/console.cfg ]; then
  # Source in any GRUB console settings if provided by the user/platform
  source $prefix/console.cfg
fi

But this volume is not present during build time. There could be a way to create a firstboot script to create such file so from 2nd boot grub is available but this is a hack. Thus creating this ticket as a reminder we should implement grub console access which is very helpful for VMs.

Start by tracing how bootupd generates the static GRUB configuration for bootc systems and how the available console.cfg path is handled during image builds. Compare that flow with the proposed firstboot approach and determine how serial console settings should be applied for VMs. Done means GRUB serial console access works through the supported boot flow rather than the ignored /etc/default/grub configuration.