Yet another set of Makefile improvements:

- Add default DEFS to -D_DEBUG
- Add --gc-sections to linker script in order to get rid of unnecessary code.
- Add print-config-verbose to list of phony targets
Rework BusDriver, such that no virtual methods are needed:

- BusDriver is only declared in enc28j60.h. Implementation is provided at
  link time.
- Provide implementation for Stellaris Launchpad in enc28j60_stellaris.(cpp|h)
- Get rid of driverId, and replace it with a void* in BusDriver.
  On call it will be the 'this'-pointer in the ENC28J60. This should be
  enough information to support multiple driver instances without
  incurring any overhead for the single instance case.
  (only overhead is the fact that the method call to the BusDriver is
   needed and the extra void* parameter).
Move everything except printf to BusDriver. This makes the driver
independent of both uIP and StellarisWare
Add doxygen configuration and initial doxygen comments
Add doxygen to .gitignore
Make ENC28J60::Init() call BusDriver::Init()
Don't use the same pins as the ENC28J60 uses
Initialize SysCtl and interrupts before initializing the ENC28J60.
Minor cleanup