Eg pyb.freq(120000000) sets the CPU to 120MHz. The frequency can be set
at any point in the code, and can be changed as many times as you like.
Note that any active timers will need to be reconfigured after a freq
change.
Valid range is 24MHz to 168MHz (but not all freqs are supported). The
code maintains a 48MHz clock for the USB at all times and it's possible
to change the frequency at a USB REPL and keep the REPL alive (well,
most of the time it stays, sometimes it resets the USB for some reason).
Note that USB does not work with pyb.freq of 24MHz.
This patch updates ST's HAL to the latest version, V1.3.0, dated 19 June
2014. Files were copied verbatim from the ST package. Only change was
to suppress compiler warning of unused variables in 4 places.
A lot of the changes from ST are cosmetic: comments and white space.
Some small code changes here and there, and addition of F411 header.
Main code change is how SysTick interrupt is set: it now has a
configuration variable to set the priority, so we no longer need to work
around this (originall in system_stm32f4xx.c).
Blanket wide to all .c and .h files. Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.
Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.
