/docs/design_guide: added links to firmware build learning guides for SAMD21 & ESP8266. Changes were placed in the "Adding native modules" section, since that seemed to me the best place based on target audience.
Updated documentation for `delay()` which fixes#243.
* Add microcontroller.cpu, the sole instance of microcontroller.Processor.
microcontroller.cpu.frequency is the clock frequency, in Hz.
microcontroller.cpu.temperature is the reading from the internal temperature sensor, in Celsius. None if not available.
* Squeeze firmware size by using -finline-limit. Otherwise non-Express builds were slightly too big.
* Update submodules.
* Fix documentation glitches
* atmel-samd: Introduce a nvm module for non-volatile byte-level memory access.
This allows for persisting small configuration values even when the file system
is read-only from CircuitPython.
Fixes#160
* Review feedback:
* Add tests.
* Fix non-zero index.
* Fix len()
* Track status pin use by user code separately so it can take over the pins and then give them back.
* Switch to hardware SPI for APA102 on Gemma and Trinket.
* Merge microcontroller/types.h into microcontroller/Pin.h to better match approach going forwards.
Docs are here: http://tannewt-micropython.readthedocs.io/en/microcontroller/
It differs from upstream's machine in the following ways:
* Python API is identical across ports due to code structure. (Lives in shared-bindings)
* Focuses on abstracting common functionality (AnalogIn) and not representing structure (ADC).
* Documentation lives with code making it easy to ensure they match.
* Pin is split into references (board.D13 and microcontroller.pin.PA17) and functionality (DigitalInOut).
* All nativeio classes claim underlying hardware resources when inited on construction, support Context Managers (aka with statements) and have deinit methods which release the claimed hardware.
* All constructors take pin references rather than peripheral ids. Its up to the implementation to find hardware or throw and exception.
