Union breakpoint

[klemens-morgenstern]

For smaller devices that only provide two breakpoints the current approach might be problematic, because when using the test-lib, the _exit-code breakpoint and the syscalls (for gcov) in conjunction we already need three. There should be a way to remedy that and reduce them to one. The most obvious way is to add a METAL_BREAKPOINT macro. When compiled in unified mode (or whatever I'll call that) it enters a call to metal_trigger_breakpoint which exists ONCE in the binary and which contains the breakpoint. Then the debugger calls step-return and calls the breakpoint it's in.