add initial beaglebone pinmux

Taking from Arduino-1.5.x, I've added a variant.cpp and
variant.h. The variant.cpp contains a preliminary pinmux for the
Beaglebone Black.

TO-DO:

- fix incompatibility of Analog Pins with structure
- a more functional and informative structure
- add support for loading overlays from init()

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>
Update boards.txt

Add variant beaglebone

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>
Add pinmux structure

The virtual_main.h file now has the pinmux structure
for defining and loading the pinmux from init().
It also has small fixes to make it work with the new files

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>
Add Arduino.h

Arduino.h for compatibility.

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>
Add initial GPIO infrastructure

Fixed wiring_digital.c to work with Userspace.
export_gpio() exports the pins in sys/class/gpio
TO-DO:
- Abstracted sys_fs functions for access to filesystem
- LED support in digitalWrite()

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>