Drivers delight - OS functions and sensor drivers

[UAVflightdirector]

These changes are to make our long term programming and user lives
much easier.
They are a reasonable step towards the elusive "plug-and-play"

1. Registerable event handlers / software interrupts
A piece of software can register a function to be run at low priority
when an event happens.

2. Registerable software timers
For doing +-1ms accuracy timing events up to 60s
Timers can be single shot or repetative
These trigger an event so that the low level function runs at the end
of the timer

3. SPI port drivers
Device independant, interrupt driven port drivers.
Many devices can be added to the same port and co-operate.
Device can easily be moved between ports depending on physical setup.
Set a communication in progress. The driver will run a callback
funciton on completion.

An I2C version of this driver is planned.

4. SCP1000 barometer drivers
Uses 1,2,3 to get pressure readings.
Note that the pressure readings do not go anywhere yet. The aim is to
do a CAN broadcast with this data.


The SPI and SCP1000 drivers are disabled in the released code build
The timers and events are being used to update the LEDs.

Matt