It was observed in regular-gnustep-20150916-*.iso that
mediacheck boot target sort of starts plymouth bootsplash
when not intended to; let's plug this by moving +plymouth
away to a test image *but* there are couple things with it:

1) supporting plymouth in mediacheck *might* be a good thing
   for distros proper -- this will likely require some support
   on bootsplash theme side either;
2) it's not enough to just drop @bootargs@ from syslinux.cfg
   snippet as the extra payload will still go in to no avail.