lualoader: improve the design of the brand-/logo- mechanism

In the previous world order, any brand/logo was forced to pull in the
drawer and call drawer.add{Brand,Logo} with the name their brand/logo is
taking and a table describing it.

In the new world order, these files just need to return a table that maps
out graphics types to a table of the exact same format as what was
previously being passed back into the drawer. The appeal here is not needing
to grab a reference back to the drawer module and having a cleaner
data-driven looking format for these. The format has been renamed to 'gfx-*'
prefixes and each one can provide a logo and a brand.

drawer.addBrand/drawer.addLogo will remain in place until FreeBSD 13, as
there's no overhead to them and it's not yet worth the break in
compatibility with any pre-existing brands and logos.

Reviewed by: freqlabs
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24966