util::shared: Fix nightly churn
util::shared: Allow dead code for NoInterrupts
util::shared: Always compile

This really isn't specific to multitasking
os::cond_var: Dust off and add single-threaded implementation
os::cond_var: Always compile
cortex: Move NoInterrupts to common
mutex: Style fixes
mutex: Single threaded implementation
mutex: Always compile
Merge pull request #194 from bgamari/concurrency

Concurrency  primitives