Remove support for booting ARTOS images

Pantelis Antoniou stated:
	AFAIK, it is still used but the products using PPC are long gone.
	Nuke it plz (from orbit).

So remove it since it cleans up a usage of env_get_char outside of
the environment code.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>