This keyword was misused due to the lack of understanding
of its meaning; thanks zerg@ for the hint, let's hide the
kernel boot parameters not expected to be modified by user
under the hood (and vice versa in some cases).

Note that this is just a keyword shift in all cases
but mediacheck where "showopts" is now added.

NB: this keyword is only relevant for gfxboot.

...just like any other stage2 based boot target,
both for consistency and specifically for propagator
test image.

It's not neccessarily installation media.

Shift boot order so that rescue image can get mediacheck too
but still boot into rescue by default.