lib/pin_change: do not implicitly initialize, move to separate file

For sake of transparency, have the user call pin_change_init()
explicitly, but then have pin_change_init() also enable interrupts for
the respective ports.

Discussed-with: @bgamari
lib/spi: use NULL for pointer
lib/pit: style changes
lib/gpio: add GPIO_DISABLE pin "direction"
include/MK20DZ10.h: drop INT_* constants to avoid confusion with our IRQ_* constants
include/smc.h: convert vllsm to enum
lib/printf: compile reduced feature version by default

Features can be selected using PRINTF_WITH in make.  Currently
supported:

- FIXPOINT:: fixed point support (kKrR)
- 64BIT:: 64-bit numeral support (ll/j)
- PAD:: numeral left/right padding support