kbuild: added vfs dependence to the device core.

The device core module will now automatically select the virtual file
system for building.

Signed-off-by: Michel Megens <dev@michelmegens.net>
sched: preempt: fixed an arithmic preemption bug.

In the scheduling core, a 64-bit integer was substracted from an
unsigned int. This caused trouble on ATmega AVR systems, since an
unsigned int is 16 bits wide on ATmega AVR.

Signed-off-by: Michel Megens <dev@michelmegens.net>