The serial output (but it's named console, not serial actually) causes
issues on U-Boot's EFI, at the very least.
This is inspired by OpenSUSE's approach:
* https://build.opensuse.org/package/view_file/Base:System/grub2/grub2-SUSE-Add-the-t-hotkey.patch
Where they add a hidden menu entry, which can be used to force the
console output.
The `echo` will be visible on the serial terminal (grub "console"),
while the graphical interface is shown. Note that input in the serial
terminal (grub "console") will continue controlling the graphical
interface. Useful if you have an SBC connectedinto an HDMI monitor, but
no keyboard connected to it.
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