This commit enables Cortex-M CPU interrupt sub-priorities
and allows the PendSV interrupt to have a priority different
from the default one. Together these two preprocessor
defines can be used to have PendSV always run as the last interrupt
before returning from the interrupt stack back to the user space.
Running PendSV as the last interrupt before returning to the
user space is recommended by ARM, as it increases efficiency.
Furthermore, that change enhances stability a lot with the
new nRF52 SoftDevice support, currently being worked in
PR #9473.
This commit merely enables sub-priorities and a separate
PendSV priority to be used without changing the default
RIOT behaviour.
Refactor cortexm_init to allow bits and pieces of
it to be called separately, while retaining the
current API, too. Needed for non-standard
Cortex-M initialisation, such as with nRF52
SoftDevice.
- added a centralized core implementation for all cortex CPUs
- moved default stack size defines to cpu.h in cortexm_common
- moved uart0 bufsize define to cpu.h in cortexm_common
- moved typed of panic_t to cpu.h in cortexm_common
