This PR updates locations for files used by the bootloader depending on
the target operating system built. The current logic does not take into
account latest versions of operating systems and makes it impossible to
build ISOs against those.
With this change it is possible to correctly build CentOS 8, Ubuntu
18.04 and Ubuntu 20.04 images.
Closes-Bug: #1916913
Change-Id: I3ed0041640f539e82805d03ba26fe46217f3ac3c