c6x: use boot_command_line instead of private c6x_command_line

Save some pointless copying of the kernel command line and just use
boot_command_line instead.

Also remove default_command_line as it is not referenced anywhere, and
the DT code already handles the default command line.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Tested-by: Mark Salter <msalter@redhat.com>
Acked-by: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: linux-c6x-dev@linux-c6x.org
Reviewed-by: Grant Likely <grant.likely@linaro.org>