MIPS: TXx9: Early command-line preprocessing

* Select board by command-line option or firmware environment variable.
* Handle "masterclk=" option.
* Add boards.h to centerize board_vec declaration.

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

 create mode 100644 include/asm-mips/txx9/boards.h