Add CAN support for F2 and F4 series.

All stm32 series currently released share the same bxCAN hardware, so share can.c among all the families.

lib/stm32/f1/can.c → lib/stm32/can.c

@@ -18,7 +18,16 @@
  */
 
 #include <libopencm3/stm32/can.h>
-#include <libopencm3/stm32/f1/rcc.h>
+
+#if defined(STM32F1)
+#	include <libopencm3/stm32/f1/rcc.h>
+#elif defined(STM32F2)
+#	include <libopencm3/stm32/f2/rcc.h>
+#elif defined(STM32F4)
+#	include <libopencm3/stm32/f4/rcc.h>
+#else
+#	error "stm32 family not defined."
+#endif
 
 void can_reset(u32 canport)
 {