A whole load of random changes...

* Cleared up serial init and made it turn off the OPn LEDs at startup
* Made the simple SPI API deal with multiple Slave Selects
* Made startup code check the first 4 memory banks (more takes too long)
* Startings off YM9938 code:
  * The MSX font has been added. Includes a shonky perl script to generate
    the asm file
  * Startings of init code for text mode 1, palette setting etc