Add a runtime option to enable/disable firmware debug output

[nils-eilers]

Debug messages are nice for debugging but decrease the transfer rate a lot especially when using U1/U2 commands. Re-flashing the device each and every time with debug messages enabled or not is very cumbersome.

edit:

What about adding an XV-option? You guessed it: V means verbosity. For a quick result, XV1/XV0 could turn the debug messages on/off completely. As an addition, we could transform the existing #ifdef DEBUG_XXX into values stored in a bitfield to switch specific debug messages on/off, such as XVCHANNEL=0,BUS=1.