Some parts used HSICLK, some used HSI. Most used NOCLK, f3 used
DISABLED. Try and move all to the shorter, simpler forms, instead of
having mixed defines for different targets for the same thing. Just
because the bits themselves are different doesn't mean we should make it
more difficult for users to port code.
Start providing async routines for all blocking routines, to make it
easier to use libopencm3 in some RTOS environments. This is not in
anyway intended to be complete, this just covers a single blocking
routine, rcc_wait_for_osc_ready. Documentation added to the top level,
and provided for all stm32 families.
