Keep default append value (aka /etc/default/grub's
GRUB_CMDLINE_LINUX_DEFAULT into {default_append} variable)

Always use the value passed as now write_grub2_sysconfig() is only
called after having offered to display the default cmdline value

On first run (during installer) or when migrating from grub-legacy or
lilo, we get the first entry's parameters

Thus resolving  mga#18952

(BIOS boot partition, thus fixing mga#18923)

in order to prevent overlapping errors when adding partitions:
(b/c libparted doesn't know that diskdrake already removed the
partitions in its memory view)
"failed to add partition #1 on /dev/vda"
"Error: Can't have overlapping partitions."
"add_partition failed"