debug funneling can be more flexible

The current debug.h assumes that the main printf is used for debugging, but 
this is bad for applications where printf is used for one purpose, and errors 
are piped to another stream.

Consider an embedded application running on ARM Cortex M microcontrollers, 
because the UART can be used for an interactive serial terminal, and the SWO 
trace pin can be used for error debugging and packet dumping.

A simple solution is to use fprintf instead of printf, or debug.h should be 
moved into the include folder instead of the src folder so that the implementer 
is encouraged to modify it.

Sorry about the flood of issues that might appear trivial, but you are pretty 
much the only open source BT stack solution that I can find.

Also, please enable the issue tracker to allow submitters to set the issue type 
to enhancement instead of defect.

Original issue reported on code.google.com by frank.zhao.main@gmail.com on 26 Jan 2014 at 5:57

mckold / btstack

debug funneling can be more flexible #372