This guide documents how one can hide the Linux boot messages from the HDMI, in case you want to have a really clean display startup option.
How to hide systemd messages at boot
To hide systemd messages from the screen (before any GUI takes over for instance), you need a couple of steps.
The first step is to remount /boot as a read-write partition because by default all files in there are protected. To do so, you need to ssh in, and execute (as the debian user):
sudo mount -o remount,rw
Then, you can edit the file with
sudo nano /boot/armbianEnv.txt
Now the file contains multiple sections, each parameter line starts with parameterName, followed by = and the values for that parameter.
The one we want to modify here is the extraargs parameter line.
To disable systemd output, make sure you have in that line (and not a new line!):
systemd.show_status=0 vt_global_cursor_default=0 quiet
When you're done, exit and save the file with Ctrl+X and enter.
Make sure to reboot as soon as possible to prevent any corruption to any of the /boot files.
Hiding the text login prompt
Depending on what application or framebuffer image you're using, you may get the occasional drop to the VT1 (virtual terminal 1) on the display, and it will show you a simple text login prompt that you may wish to hide.
To do so, you simply need to run:
sudo systemctl mask getty@tty1