prepare-logo: Add support of mx8 boot.cfg
A MR for generic "bootlogo mx8mp" task, which was provided without specific ID number.
i.MX8 machines do not use the FNG-Boot, they use the imx U-Boot instead. But boot.cfg is still used to get cmdline. And the config format is slightly different for i.MX6 and i.MX8.
The commit changes the sed command to support both flavors of configs.
Edited by Dmitry Petrov