Added support for power gating the DS2411

I added an optional Ds24x1PowerControlC module that provides a
stdcontrol interface to allow platforms to start and stop the ds2411.
The version in tos/chips/ds24x1 is just a dummy version that doesn't do
anything, but a platform can override it and make the start() and stop()
commands more interesting.

To add this to the shimmer platform I created the Ds24x1PowerControlC/P
component in the shimmer platform, and that wires to and controls pin
2.3. Adding that means that shimmer2/2r/span can no longer borrow
PlatformIeeeEui64.h from shimmer without also getting the power control
module, so I had to replicated that for the 2 and span platforms.

Finally I removed the macro from shimmer/MotePlatformC that asserts pin
2.3 at boot.