The IPSR (=Interrupt Program Status Register) contains the exception
type number of the current Interrupt Service Routine. Printing this
out can be useful to detect if a certain interrupt is not implemented.

- removed coretx-m[0|3|4]
- moved their content to cortexm_common
- adjusted cortex Makefiles to this new name
- adjusted cortex Makefiles new structure

Change from `void reboot(void)` to `int reboot(int mode)`.
Move reboot definition to core, rename architecture implementations
from reboot to reboot_arch.
Declare reboot mode(s) in kernel.h, reboot_arch in kernel_internal.h
Currently only one reboot mode is handled, its use is enforced.

Rationale:
A reboot function is already defined in <unistd.h> on BSD systems.
(See: http://www.openbsd.org/cgi-bin/man.cgi?query=reboot&sektion=2)
This patch not only allows native to build sensibly on these systems
but also streamlines RIOTs compatability with existing software.