Update sysfs.h and sysfs.c

Tweaked the sysfs functions a little

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

- digitalWrite and digitalRead() now use sysfs abstractions!
- removed export_gpio() from wiring_digital.c
- Small tweaks to make code work

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