Otherwise, use/branding is pointless in itself.

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.

As duly noted by glebfm@, branding issues need more attention
by now since only stage1/install2 got some of it so far in this
regard.  Hence the dedicated feature comes to the rescue
(well no, it doesn't actually mess with rescue!).