wiiu: Fixup FB pixel format.

linux-loader places the FB in a strange format, let's restore it to
something more reasonable. Retire the ridiculous "is_brg" option.

wiiu: Fix parsing of multiple cmdline options.

wii_cmdline (inherited from HBC) expects options to be separated by NUL,
with an extra NUL character at the end.

Add support for the Nintendo Wii U.