nrf52840: add support for flashing with the BOSSA tool

This only works with a custom bossac build from Arduino, not with the
upstream version. It avoids needing the manual "double tap" to enter
bootloader mode before flashing firmware.

src/machine/machine_nrf52840_usb_reset_bossa.go

@@ -0,0 +1,26 @@
+// +build nrf52840,nrf52840_reset_bossa
+
+package machine
+
+import (
+	"device/arm"
+	"device/nrf"
+)
+
+const DFU_MAGIC_SERIAL_ONLY_RESET = 0xb0
+
+// checkShouldReset is called by the USB-CDC implementation to check whether to
+// reset into the bootloader/OTA and if so, resets the chip appropriately.
+func checkShouldReset() {
+	if usbLineInfo.dwDTERate == 1200 && usbLineInfo.lineState&usb_CDC_LINESTATE_DTR == 0 {
+		EnterSerialBootloader()
+	}
+}
+
+// EnterSerialBootloader resets the chip into the serial bootloader. After
+// reset, it can be flashed using serial/nrfutil.
+func EnterSerialBootloader() {
+	arm.DisableInterrupts()
+	nrf.POWER.GPREGRET.Set(DFU_MAGIC_SERIAL_ONLY_RESET)
+	arm.SystemReset()
+}

src/machine/machine_nrf52840_usb_reset_none.go

@@ -1,4 +1,4 @@
-// +build nrf52840,!nrf52840_reset_uf2
+// +build nrf52840,!nrf52840_reset_uf2,!nrf52840_reset_bossa
 
 package machine
 

targets/nano-33-ble.json

@@ -1,6 +1,6 @@
 {
 	"inherits": ["nrf52840"],
-	"build-tags": ["nano_33_ble"],
+	"build-tags": ["nano_33_ble", "nrf52840_reset_bossa"],
 	"flash-command": "bossac_arduino2 -d -i -e -w -v -R --port={port} {bin}",
 	"flash-1200-bps-reset": "true",
 	"linkerscript": "targets/nano-33-ble.ld"