[arch] Add __drivers and __devices arrays to ld scripts.
[dev] Add simple multi-instance device driver model.

Model supports class drivers capable of driving multiple device
instances independently. Implements simple pattern for device life
cycle management. Completely optional and self-contained.
[platform][sam3] Start getting TWI and SPI drivers working.

Drivers use the driver model to support multiple controller instances.
[target][sam3x-ek] Configure and init TWI and SPI drivers.
[git] Add cscope.* to .gitignore.
[kernel] Keep load percentage from underflowing.
WIP - pc driver model uart and ide drivers; openocd-sam3x