RFC: Return number of bytes written in M packet

[Andrew Cagney]

Hello,

This is intended to close what I think is a hole in the current memory 
write packet - there isn't a way that a target can report back a partial 
(i.e. fragmanted) write.

The change is simple.  As well as:

	<- "OK"
	<- ""
	<- "Enn"

allow:

	<- <BE-hex-nr-bytes-written>

as a reply.

Memory reads already allow partial transfers.

	Andrew