stackit-oem: Add custom grub config to configure linux_console

[RAPSNX]

stackit-oem: Add custom grub config to configure linux_console

This changes the grub.cfg of the stackit-oem image, in STACKIT it is possible to retrieve logs from the serial-console. (/dev/TTYS0).
Many services are logging by default to the system-console (/dev/console).
In order to link the serial-console to the system-console, the last parameter of the cmd_line arguments is used.

Basically, this switches the default parameter for the linux_console.

How to use

The resulting image should have the console=ttyS0,115200n8 param as last console= param set.
This can be verified by checking the cmd_line:

cat /proc/cmdline

Testing done

• Build the image from source according to sdk-modifying-flatcar
• Uploaded the image into openstack and created a server according to stackit-docs
• Checked that the cmdline has the correct setting