Comment 32 for bug 797544

For me... solved it by noticing that after I use
sudo umount /boot
the command
ls /boot
still showed directory content. Therefore, every adjustment to the file /etc/default/grub would be written to the directory on /dev/sda1 that is mounted over my boot directory on /dev/sda3. However, during boot, mounts won't take place after a selection is made in the grub menu (either manual or by timeout)

So, I remounted my partition:
sudo mount /boot
and noted the partition I made long time ago as a boot partion by issuing the command:
mount|grep boot
I then unmounted the boot partition, moved the directory out of the way, and made a new mount point, and mounted again by:
sudo umount /boot && sudo mv /boot /boot.backup && sudo mkdir /boot && sudo mount /boot
After that, I installed the correct info to the mbr (master boot record):
sudo grub-install --recheck /dev/sda
Edited the configuration file /etc/default/grub and ran update-grub.

Hope this helps people who stumble upon this bug looking for a solution. Bear in mind you only try these commands when you know what each command does, and understand the underlying logic. I leave it to you to determine if you use MBR or uefi, or want a unbootable system.