efi_loader: Avoid overwriting previous outputs on console screen clearing

Before clearing the screen, ensure that no previous output of firmware
or UEFI programs will be overwritten on serial devices or other
streaming consoles. This helps generating complete boot logs.

Tested regarding multi-output against qemu-x86_defconfig. Still, there
were remaining concerns about side effects, so this is provided as an
opt-in feature.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>