At boot time, GRUB on Debian and most other distros will show something like:

GRUB loading.
Weclome to GRUB!

and then load the boot menu.

By editing /etc/default/grub, you can easily disable the boot menu. However, to disable those two messages, you have to patch grub and recompile, which is cumbersone, as it forces you to maintain your own .deb files up-to-date with the needed patches.

grub-shusher contains two tiny .c files that will patch your master boot record and grub files to disable those two messages.

The software is as safe as I could make it: it looks for a specific set of patterns, and if not all are found, it stops processing. I have tested it on a few machines, and it is working.

Consider though that they will read your master boot record and modify it. This probably does not work on EFI systems.

USE THEM AT YOUR OWN RISK

ONLY TESTED on AMD64 using BIOS BOOT - NO EFI

REPLACE /dev/sda with your GRUB PARTITION, used with grub-setup or grub-install

$ make
$ sudo -s
# ./grub-kernel /boot/grub/kernel.img
# ./grub-kernel /usr/lib/grub/i386-pc/kernel.img
# grub-install /dev/sda
# ./mbr /dev/sda

... and done. Note that the order is important:

1. make will compile the code, you need to have GCC installed.
2. grub-kernel ... will remove the 'Welcome to GRUB!' message from the kernel.img file.
3. grub-install /dev/sda will create a new compressed image (by merging several other files, including kernel.img) and install it on your disk.
4. mbr /dev/sda will remove a few other messages from the installed mbr.

To make GRUB entirely quiet, my /etc/defaults/grub has:

GRUB_DEFAULT=0
GRUB_TIMEOUT=0
 
GRUB_HIDDEN_TIMEOUT=5
GRUB_HIDDEN_TIMEOUT_QUIET=true

Note that once you have those lines, you need to run update-grub, and on next reboot, you will have to press 'ESC' or keep 'shift' pressed to get into the grub menu. I suggest you try this before you shush grub.

If something goes wrong, you can:

# apt-get install --reinstall grub2
# grub-install /dev/sda

to clean up after yourself.

You can read more about grub-shusher on this blog post.