stm32f0: use usart-v2 instead of private usart

Use the usart-common base plus the usart-v2 code, instead of private
implementations.  Less code, more common apis across targets.

Of note is the trick to make F0 look like it has an APB2 bus.  It's the
only stm32 that doesn't have a documented APB2 bus, but still has
peripherals enabled via an "APB2" register, and they match how other
targets have an APB2.  Simply make APB2 an alias of APB1, as it's only
used for clock speed detection.

include/libopencm3/stm32/f0/rcc.h

@@ -389,6 +389,10 @@ Control</b>
 /* --- Variable definitions ------------------------------------------------ */
 extern uint32_t rcc_ahb_frequency;
 extern uint32_t rcc_apb1_frequency;
+/** F0 doens't _realllly_ have apb2, but it has a bunch of things
+ * enabled via the "APB2" enable register. Fake it out.
+ */
+#define rcc_apb2_frequency rcc_apb1_frequency
 
 enum rcc_osc {
 	RCC_HSI14, RCC_HSI, RCC_HSE, RCC_PLL, RCC_LSI, RCC_LSE, RCC_HSI48