Build kernel using relocatable AOF and support multiple source files
This enables C code to be incorporated into the Kernel.
- Add KernelBaseA absolute symbol.
- Use KernelBase - KernelBaseA to convert some expressions to/from AREA relative form.
- Link to correct address.
- Remove ORG directive
- Move EndOfKernel to separate AREA to allow linked code in between.
- Adjust Makefile link multiple object files.
Requires !34 (merged).