Carry on working on SAMA5D3 demo:

- Add full interrupt nesting tests.
- Add additional critical section/context switching tests.
- Set interrupt priorities so everything can run at once without any software watchdog errors.
- Re-enable interrupts in each IRQ handler.
- Add in run-time stats.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2282 1d2547de-c912-0410-9cb9-b8ca96c0e9e2