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
util::support: Add __aeabi_memset

This is helpful to have when compiling without optimization
Merge pull request #198 from bgamari/misc

Miscellaneous fixes
nvic: Port to ioregs!
scb: Port to ioregs!
mpu: Port to ioregs!
Merge pull request #197 from bgamari/nvic-ioregs

Port NVIC to ioregs!
systick: Kill reference to old ioreg
Merge pull request #201 from bgamari/nvic-ioregs

systick: Kill reference to old ioreg
nvic: Add comments and STIR
Merge pull request #202 from bgamari/nvic-ioregs

nvic: Add comments and STIR
k20::i2c: Initial commit