Added standard clock setup routines.

Thanks to Thomas Otto for pointing out problems with the clock code in
examples and his clock routine implementations. Based on that the most
common clock combination routines were added to the library and all
routines in examples setting up the clock replaced with calls to that
functions.

include/libopenstm32/rcc.h

@@ -398,5 +398,7 @@ void rcc_set_ppre2(u32 ppre2);
 void rcc_set_ppre1(u32 ppre1);
 void rcc_set_hpre(u32 hpre);
 u32 rcc_get_system_clock_source(int i);
+void rcc_clock_setup_in_hsi_out_64mhz(void);
+void rcc_clock_setup_in_hse_8mhz_out_72mhz(void);
 
 #endif