reelboard: add SoftDevice target reelboard-s140v7

Unfortunately, `tinygo flash` doesn't work here. You have to merge the
SoftDevice and the application hex with a mergehex tool and flash that
to the board.

Makefile

@@ -289,6 +289,8 @@ smoketest:
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=pca10056-s140v7     examples/blinky1
 	@$(MD5SUM) test.hex
+	$(TINYGO) build -size short -o test.hex -target=reelboard-s140v7    examples/blinky1
+	@$(MD5SUM) test.hex
 ifneq ($(AVR), 0)
 	$(TINYGO) build -size short -o test.hex -target=atmega1284p         examples/serial
 	@$(MD5SUM) test.hex

targets/reelboard-s140v7.json

@@ -0,0 +1,3 @@
+{
+	"inherits": ["reelboard", "nrf52840-s140v7"]
+}