#3 Header file cleanup

[Bob Armstrong]

The header file situation needs a little clean up. There are some header files (e.g. standard.h) which are shared by both the apu and mcu - these should be put in a common directory to avoid duplicate copies.

Also, the APU message declarations (for messages exchanged between the APU and MCU) are in the file apu.h in the mcu project, but then are duplicated in the file host.h in the apu project. These should be combined into one file so that there's only a single set of definitions and then put into a common place.

The ps2.asm file (in apu) should use a reglpx052 include file instead of regx051, but I didn't have one. reglpx052 needs to be created and then regx051 can be removed from the project.