Merge pull request #68 from farcaller/pt-lpc

Refactor PlatformTree

Reviewed-by: farcaller,me
Better spelling for error messages.
Moved init_stack to hal::mem_init.
hal::lpc17xx::init::init_clock is now public.
Fixed pll struct generation.
build_mcu for lpc17xx no longer emits init_stack/init_data.
Builder now emits main, __morestack and task_scheduler.
platform tree!() macro is now functional.
Build everything with debug symbols.
Refactoring app_empty, pass 1.
node::PlatformTree stores nodes by path, allows anonymous.
More cleanups and fixes.
Added node::Node.expect_subnodes.
Switched to &[&str] from Vec<&str>.
Added basic os/single_task PT, no tests.