Simple Hello World for 68HC11 bootstrap mode

This 68HC11 bootstrap program acts as a simple server for uploading a program in external memory. It supports only two commands:

Implementation Notes:

1. This program must not be linked with a startup file. It implements the startup entry point.
2. The _start function must be at beginning of this file. By doing so, it will be mapped at address 0 by the linker and we avoid to have some jump to call it (since the boot will jump there).
3. It must be compiled with -mshort -Os -fomit-frame-pointer to make sure it fits in less than 240 bytes (keep a few bytes at end for the stack).
4. It must be compiled with -msoft-reg-count=0 or =1 to reduce the use of soft-registers (since they are mapped in RAM in .page0).
5. The soft registers must be located at beginning of the .page0. This will clobber the beginning of the _start function. We don't care because this clobbers some initialisation part.

Source file: hello.c