Make FreeRTOS work on Raspberry Pi 3

[smolt]

Make FreeRTOS work on Pi 3 - and maybe Pi 2 while we are at it.

There is currently only support for the original Pi . Known difference are peripheral base addresses and access to on board activity LED has been moved to an I/O extender. Information can be found on how to access the LED: https://www.raspberrypi.org/forums/viewtopic.php?f=72&t=139684.

Demo of using the on board Pi3 LED is here: https://github.com/SatelliteCore/RPiDemo/blob/master/pi3/rpi3blinkc.c

[smolt]

Got FreeRTOS working on Pi 3, blinking the onboard LED with S-O-S morse code. The problem was that the latest Pi firmware boots the ARM processor into Hypervisor mode, but FreeRTOS expects Supervisor mode. Trick was to get back into Supervisor mode before launching FreeRTOS.

I will update this repository later, but for now the changes that work on Pi 3 can be found at:

https://github.com/smolt/RaspberryPi-FreeRTOS

[smolt]

Created PR #2 as I don't have access to directly commit to this repository. @elektrux can pull the change.

[elektrux]

@smolt (+ @mingynaut ) gave everyone owner permissions, I thought I did that a ways back.