Debug: reduce need for program buffer words

[mwachs5]

Currently the way FESVR uses the program buffer to load programs and write CSRs, it needs a lot of words of program buffer and doesn't consider implicit ebreak. (It doesn't handle the case if an implementation uses abstract commands instead of program buffer, but that's of a lesser concern). OpenOCD can do it with 2 words of program buffer + implicit ebreak, or 3 words of program buffer, fesvr should be able to do the same.