Determine path to libgcc.a automatically
Merge pull request #75 from errordeveloper/detect_libgcc_path

Determine path to libgcc.a automatically

Reviewed-by: farcaller
Update readme with useful links to website.
Merge pull request #76 from farcaller/update-readme

Updated readme with useful links to website.

Reviewed-by: farcaller
Renamed UIntValue to IntValue (still uint though).
Added platformtree::node api docs.
Split platformtree::builder into parts.
Node::attributes is now wrapped in RefCell.
mcu::clock PT node now provides calculated `system_frequency` attribute.
gpio PT node now supports specifying a pin function.
Renamed node::StringAttribute to StrAttribute.
Fixed AltFunction indexing.
Old pinmap is gone, so is mbed board that used to depend on it.
Ported UART demo to PT. Broken, as PT now needs cross-node dependency tracking.
Boards are gone
Cleaning up lpc17xx clock init code
Converted TokenString to tuple struct for simpler usage
Wrapped PLL0 configuration into Option
Renamed init to system_clock
Muted the dead code warning in ssp