The grub.cfg has two variables [1]
GRUB_CMDLINE_LINUX : used on all boots
GRUB_CMDLINE_LINUX_DEFAULT : additionally used on all "normal" boots
The problem with I2298675dda1f699c572b3423e7274bc8bd7c1c9d is that it
appened the values in DIB_BOOTLOADER_DEFAULT_CMDLINE to both of these,
resulting in duplicated arguments. I don't think we considered that
GRUB_CMDLINE_LINUX_DEFAULT actually already appends to the
GRUB_CMDLINE_LINUX values.
Make DIB_BOOTLOADER_DEFAULT_CMDLINE only append itself to
GRUB_CMDLINE_LINUX_DEFAULT. That seems to line up sensibly with the
name of the variable.
Documentation is enhanced around this, and a releasenote added.
[1] https://help.ubuntu.com/community/Grub2/Setup
Change-Id: I76b5442a9090c19a6540ed2d4ab324546f241ebf
Closes: #1791736
