um: console: Ignore console= option

Ignore linux kernel's console= option at uml's console
option handler. Since uml's con= option is only for
setting up new console, and Linux kernel's console=
option specify to which console kernel output its
message, we can use both option for different purpose.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>