I don't think we're gonna like plymouth over rescue image
anytime soon, especially when it hides the moment when shell
pops up somewhere under it without startup-rescue caring to
remove the splash.

So let's put that $(INSTALL2_BRANDING) into proper stage2
flavours only and avoid choking on missing plymouth as well.

The initial work covered live images but missed an installer bit
(thus notes and slideshow were missing in install2) while forgetting
to put branding packages into base list (thus kindly making these
available for *manual* installation sometime after, ouch).