Move pin-draining function to device specific file

[mmoskal]

This isn't the cleanest, but slightly better than current

[jamesadevine]

@mmoskal makes sense - however i'm not keen on the use of _pin which is an mbed specific internal type. Perhaps an accessor would be better incase we moved away from mbed in the future? Something like getPinName()?

[mmoskal]

Sure, but this PR doesn't do it, it was already there - I'm only using pinID which is a number

[finneyj]

yup - thanks @mmoskal. Better than where it was for sure - I squeezed this in for Peli last night and didn't think anyone would notice until I had chance to move. i should have known your eagle eyes would pick it up. :-)

I was thinking we'd end up with a platform specific implementation of the Pin class (one per platform), and we can drop it in there along with getDigitalValue() etc. as chargePin() drainPin() etc. etc.

[pelikhan]

@finneyj What's the status on this PR?