external/mambo: allow CPU targeting for most debug utils

Debug util functions target CPU 0:0:0 by default Some can be
overidden explicitly per invocation, and others can't at all.
Even for those that can be overidden, it is a pain to type
them out when you're debugging a particular thread.

Provide a new 'target' function that allows the default CPU
target to be changed. Wire that up that default to all other utils.
Provide a new 'S' step command which only steps the target CPU.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>