[RFA]: hw-glue-bus-mux selection behavior

[Dave Brolley]

Hi,

It seems more natural to me, and turns out to be more conveniant in many 
cases to have the "switch" pin of the hw-glue-bus-mux component act as a 
selector rather than as a command to switch buses. That is, when the pin 
is driven 0, then "downstream1" is selected and when the pin is driven 
1, then "downstream2" is selected. This is because the input to a 
bus-mux is frequently the value of a bit in a control register of the 
component driving the pin. It also scales well to a possible future 
bux-mux which may have more than one output accessor.

Jeff Johnston, the original author of the component has agreed with me 
in principal and so, I present this patch for approval. Anyone currently 
using this component would have to make changes to their code.

Notes:
1)  I actually implemented the selector to choose "downstream1" when the 
pin value is even and "downstream2" otherwise. This works well for 0 and 
1 values and provides a way for a component to toggle between the 
busses, as before, by simply incrementing the pin value. Once again, 
this scales to a multi-bus-mux where one could rotate between the busses 
by incrementing the pin value.

2) I left the mux_passthrough_bus::switch_bus function in since it still 
seems like useful functionality for other subclasses.

OK to commit?

Dave