spi: Add spi_is_idle

This allows us to safely determine when it's safe to sleep the core
spiflash: Remove spiflash_is_idle

This wasn't safe
usb: Enable RESUME support

This allows capable firmware to enter sleep modes which de-clock the USB
module.
Merge branch 'master' of https://github.com/mchck/mchck
Merge branch 'master' of https://github.com/mchck/mchck
Merge branch 'master' of https://github.com/mchck/mchck
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
nrf: Call pin_change_init
Merge pull request #103 from bgamari/master

nrf: Call pin_change_init

This was overlooked in 3397b6b4520aeaa4ebfc52f92552662908accf6f.
Add pmtest
pmtest: Disable USB regulator
pmtest: Cleanup
pmtest: Modify to cycle through power modes
Port to pin_change infrastructure