To ensure dracut does not load nouveau we need to explicitly disable it via
omit_drivers.
This change adds a method to drop in arbitary dracut conf files to an element
which are picked up by dracut-regenerate and included in the chroot where we
run dracut.
The disable-nouveau element just adds a conf file with
`omit_drivers += " nouveau"`
The default dracut conf files in /usr/lib include a similar file to omit the
nvidia kernel modules.
Change-Id: I6375e4843fd08d1410141fbbd8658042dcd5ad05
Closes-bug: 1842664
