MIPS: TXx9: Cleanup builtin-cmdline processing

Since commit 898d357b5262f9e26bc2418e01f8676e80d9867e (lmo) /
6acc7d48

 (kernel.org) ("Fix and enhance
built-in kernel command line") arcs_cmdline[] does not contain built-in
command line.  The commit introduce CONFIG_CMDLINE_BOOL and
CONFIG_CMDLINE_OVERRIDE to control built-in command line, and now we can
use them instead of platform-specific built-in command line processing.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Patchwork: http://patchwork.linux-mips.org/patch/802/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>