machine/stm32f103xx: allow board specific UART usage

Motivation: The bluepill uses USART1 as UART0 but other boards like the
STM32 Nucleo boards (and disco as well) use USART2 for USB COM port.

To avoid duplication of code the same pattern as in `machine_atsamd21.go`
is applied where only UART-specific code is moved to `board_*.go`.

src/machine/board_bluepill.go

@@ -2,6 +2,8 @@
 
 package machine
 
+import "device/stm32"
+
 // https://wiki.stm32duino.com/index.php?title=File:Bluepillpinout.gif
 const (
 	PA0  = portA + 0
@@ -47,10 +49,28 @@ const (
 
 // UART pins
 const (
-	UART_TX_PIN = PA9
-	UART_RX_PIN = PA10
+	UART_TX_PIN     = PA9
+	UART_RX_PIN     = PA10
+	UART_ALT_TX_PIN = PB6
+	UART_ALT_RX_PIN = PB7
 )
 
+var (
+	// USART1 is the first hardware serial port on the STM32.
+	// Both UART0 and UART1 refer to USART1.
+	UART0 = UART{
+		Buffer: NewRingBuffer(),
+		Bus:    stm32.USART1,
+		IRQVal: stm32.IRQ_USART1,
+	}
+	UART1 = &UART0
+)
+
+//go:export USART1_IRQHandler
+func handleUART1() {
+	UART1.Receive(byte((UART1.Bus.DR.Get() & 0xFF)))
+}
+
 // SPI pins
 const (
 	SPI0_SCK_PIN  = PA5