thus far, config bootloader validation only did the basic check that each
bootloader specified was a known and supported bootloader, it did not check
combinations.
it now checks combinations, and strips out the previous "bootloader role"
stuff.
the no-bootloaders warning is duplicated, covering two slightly different
situations (empty string, and whitespace string). this is anticipated to
be just temporary, with this just being the first step in better handling
bootloader selections.
Gbp-Dch: Short
3f840aeeba