ioreg: Support register widths other than 32 bits
stm32f4: Update ioreg usage
k20: Add linker scripts
k20::isr: Initial commit

Shamelessly stolen from Ben Harris's k20 branch.
k20: Add build system support
k20::sim: Initial commit
k20::pin: Initial commit
k20::uart: Initial commit
lpc17xx::uart: Shuffle register constants
cortex_m0: Add missing mod.rs
Share systick between cortex_m*
systick: Note that not all implementations support CALIBRATED
blink_k20: Initial commit
systick: Mark reg as public
cortex_m4/mod.rs: Add documentation
cortex_m0/mod.rs: Add documentation
Share mpu between m3 and m4
Share nvic between m0, m3 and m4
Share scb between m0, m3, and m4
Rakefile: Just build blink_k20 when PLATFORM=k20