Comment 4 for bug 1854135

The charm is not just adding the GRUB configuration but also erasing the kernel options MAAS configured.

/etc/default/grub.d/90-sysconfig.cfg rendered file contains:

```
# Juju generated file - do not edit manually

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'

# Evaluate grub-config-flags first, in case it uses $GRUB_CMDLINE_LINUX_DEFAULT.
# However, if grub-config-flags is not specified, ignore that (current behavior.)
GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT console=ttyS0,115200 console=tty0"
# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console
GRUB_TERMINAL=serial
```

Also, it adds "console=ttyS0,115200 console=tty0" to the options (which has already been added by MAAS)

IMHO, the template shouldn't set GRUB_CMDLINE_LINUX_DEFAULT="" even if the grub-config-flags option is not set. (https://git.launchpad.net/charm-sysconfig/tree/src/templates/grub.j2#n12)