machine/stm32f103xx/nucleo-f103rb: add support for NUCLEO-F103RB STM32F1-based board

Compared to the already supported stm32f103xx "bluepill" board this:

- features 128 KiB flash memory size ("RB" suffix) instead of 64 KiB, see `targets/stm32f103rb.ld`
- has onboard ST-LINK/V2-1 programmer and debugger requiring different OpenOCD configuration file
- uses USART2 connected to ST-LINK/V2-1 debugger as virtual COM port over USB for `putchar()`
- has a user-accessible button besides the reset button

src/machine/board_nucleof103rb.go

@@ -0,0 +1,124 @@
+// +build nucleof103rb
+
+package machine
+
+import "device/stm32"
+
+const (
+	PA0  = portA + 0
+	PA1  = portA + 1
+	PA2  = portA + 2
+	PA3  = portA + 3
+	PA4  = portA + 4
+	PA5  = portA + 5
+	PA6  = portA + 6
+	PA7  = portA + 7
+	PA8  = portA + 8
+	PA9  = portA + 9
+	PA10 = portA + 10
+	PA11 = portA + 11
+	PA12 = portA + 12
+	PA13 = portA + 13
+	PA14 = portA + 14
+	PA15 = portA + 15
+
+	PB0  = portB + 0
+	PB1  = portB + 1
+	PB2  = portB + 2
+	PB3  = portB + 3
+	PB4  = portB + 4
+	PB5  = portB + 5
+	PB6  = portB + 6
+	PB7  = portB + 7
+	PB8  = portB + 8
+	PB9  = portB + 9
+	PB10 = portB + 10
+	PB11 = portB + 11
+	PB12 = portB + 12
+	PB13 = portB + 13
+	PB14 = portB + 14
+	PB15 = portB + 15
+
+	PC0  = portC + 0
+	PC1  = portC + 1
+	PC2  = portC + 2
+	PC3  = portC + 3
+	PC4  = portC + 4
+	PC5  = portC + 5
+	PC6  = portC + 6
+	PC7  = portC + 7
+	PC8  = portC + 8
+	PC9  = portC + 9
+	PC10 = portC + 10
+	PC11 = portC + 11
+	PC12 = portC + 12
+	PC13 = portC + 13
+	PC14 = portC + 14
+	PC15 = portC + 15
+
+	PD0  = portD + 0
+	PD1  = portD + 1
+	PD2  = portD + 2
+	PD3  = portD + 3
+	PD4  = portD + 4
+	PD5  = portD + 5
+	PD6  = portD + 6
+	PD7  = portD + 7
+	PD8  = portD + 8
+	PD9  = portD + 9
+	PD10 = portD + 10
+	PD11 = portD + 11
+	PD12 = portD + 12
+	PD13 = portD + 13
+	PD14 = portD + 14
+	PD15 = portD + 15
+)
+
+const (
+	LED         = LED_BUILTIN
+	LED_BUILTIN = LED_GREEN
+	LED_GREEN   = PA5
+)
+
+const (
+	BUTTON      = BUTTON_USER
+	BUTTON_USER = PC13
+)
+
+// UART pins
+const (
+	UART_TX_PIN     = PA2
+	UART_RX_PIN     = PA3
+	UART_ALT_TX_PIN = PD5
+	UART_ALT_RX_PIN = PD6
+)
+
+var (
+	// USART2 is the hardware serial port connected to the onboard ST-LINK
+	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
+	// Both UART0 and UART1 refer to USART2.
+	UART0 = UART{
+		Buffer: NewRingBuffer(),
+		Bus:    stm32.USART2,
+		IRQVal: stm32.IRQ_USART2,
+	}
+	UART2 = &UART0
+)
+
+//go:export USART2_IRQHandler
+func handleUART2() {
+	UART2.Receive(byte((UART2.Bus.DR.Get() & 0xFF)))
+}
+
+// SPI pins
+const (
+	SPI0_SCK_PIN  = PA5
+	SPI0_MISO_PIN = PA6
+	SPI0_MOSI_PIN = PA7
+)
+
+// I2C pins
+const (
+	SCL_PIN = PB6
+	SDA_PIN = PB7
+)

src/machine/board_stm32.go

@@ -1,4 +1,4 @@
-// +build bluepill stm32f4disco
+// +build bluepill nucleof103rb stm32f4disco
 
 package machine