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
k20: Fix linker script