Organizational updates. Mega2560 support moved.

- Mega2560 support has been moved to the Grbl-Mega
[project](http://github.com/gnea/grbl-Mega/) to clean up the code and
make future developments easier with increased flash and RAM. All new
developments between the 328p and Mega2560 will be synced when it makes
sense to.

- OEM single file compile configuration option. Before OEMs needed to
alter three files. Provided a way to just alter the config.h file to
contain everything for a particular Grbl build. See config.h for more
details.

- Removed defaults and cpu_map directories and reverted back to
defaults.h and cpu_map.h to contain all definitions. This should help
reduce some headaches and the previous implementation inadvertently
created. Also, it makes the single file config.h possible.

- Moved (and tweaked) the invert control pin mask define and placed
into config.h, rather than in the cpu_map.h file. Makes more sense
there.

README.md

@@ -42,6 +42,8 @@ Grbl includes full acceleration management with look ahead. That means the contr
 
 - Updated G28 and G30 behavior from NIST to LinuxCNC g-code description. In short, if a intermediate motion is specified, only the axes specified will move to the stored coordinates, not all axes as before.
 
+- **NOTE:** Arduino Mega2560 support has been moved to an active, official Grbl-Mega [project](http://www.github.com/gnea/grbl-Mega/). All new developments here and there will be synced when it makes sense to.
+
 - A few bug fixes and lots of refactoring to make the code more efficient and flexible.