When booting RPi3 image without a monitor connected, the system does not show
any output on the serial console when starting grub. As a result we can't
change the kernel console parameter neither.

Expected behaviour:
A message like:
"Please press 't' to show the boot menu on this console"
should be shown on the serial console that will allow us to switch to serial
console grub menu.

Actual behaviour:
No output from grub2 on the serial console. Kernel get's started without the
posibility to interact with grub2 through the serial console.

Tested with:
openSUSE-Tumbleweed-JeOS.aarch64-15.1.0-RaspberryPi-Build9.29.raw.xz from
Virtualization:Appliances:Images:openSUSE-Tumbleweed/kiwi-templates-JeOS