Merge pull request #196 from bgamari/misc

Miscellaneous warnings fixes
cond_var: Use wfi() instead of asm!("wfi")

Forgot the "volatile" in the open coded version
cond_var: Fix inverted logic; a race condition remains
Use -C no-stack-check instead of #[no_stack_check]
util::support: Add __aeabi_memset

This is helpful to have when compiling without optimization
Add debug tracing module