This system makes it a lot easier to include external libraries as static,
native modules in MicroPython. Simply pass USER_C_MODULES (like
FROZEN_MPY_DIR) as a make parameter.
Move hardware-specific optimizations to the very end of document, and
add visible note that it gives an example for Pyboard. Remove references
to specific hardware technologies, so the doc can be more naturally
used across ports. Various markup updates to adhere to the latest
docs conventions.
