MIPS: TXx9: Improve handling of built-in and command-line args

* Make prom_init_cmdline() static and be called from prom_init.
* Append built-in args if the first character was '+'.
* Drop command-line args if the first character of built-in was '-'.
* Enclose args include spaces by quotes.
* TX4938_NAND_BOOT is no longer needed.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>