Add test in common headers for improper inclusion of such files in application.
(prevents inclusion and also issues warning)
Changes to some source files to remove references to common headers.
Changes to rng.h to make guard symbols the same in f2 and f4.
This mainly moves the STM32 timers' code to the common area.
F2 and F4 have a common section to deal with the options register (TIM2 and TIM5 only)
L1 has been made common with timer_common_all as its options register has very different settings to F2/F4. Code is in the L1/timer.c L1/timer.h files

Note that F3 and F05 timers should fit into this scheme, with F3 having additional features.

Bundled with this is L1/pwr.h to change a documentation setting
Also all the Doxyfiles have added "ENABLE_PREPROCESSING = NO" to fix a problem introduced by commit 118.