Add flash-bmp target to Makefile

I have a Black Magic Probe now, so I'd like to be able to use that to
flash the PD Buddy Sink.  This commit adds a Makefile rule to automate
the very long command to do that.  I didn't bother with a GDB script as
the BMP wiki suggests, since it seems to add little advantage when the
command is already hidden away in a Makefile.

Makefile

 
 flash-openocd-stlink: $(BUILDDIR)/$(PROJECT).elf
 	openocd -f interface/stlink-v2.cfg -c "transport select hla_swd" -f target/stm32f0x.cfg -c "program $(BUILDDIR)/$(PROJECT).elf verify reset exit"
+
+GDB ?= $(TRGT)gdb
+
+ifeq ($(BMP_PORT),)
+BMP_PORT_CANDIDATES := $(wildcard \
+/dev/serial/by-id/usb-Black_Sphere_Technologies_Black_Magic_Probe_*-if00 \
+/dev/cu.usbmodem*1)
+ifeq ($(words $(BMP_PORT_CANDIDATES)),1)
+BMP_PORT := $(BMP_PORT_CANDIDATES)
+else
+BMP_PORT = $(error Black Magic Probe gdb serial port not found, please provide the device name via the BMP_PORT variable parameter$(if \
+$(BMP_PORT_CANDIDATES), (found $(BMP_PORT_CANDIDATES))))
+endif
+endif
+
+flash-bmp: $(BUILDDIR)/$(PROJECT).elf
+	$(GDB) -nx --batch \
+		-ex 'target extended-remote $(BMP_PORT)' \
+		-ex 'monitor swdp_scan' \
+		-ex 'attach 1' \
+		-ex 'load' \
+		-ex 'compare-sections' \
+		-ex 'kill' \
+		$(BUILDDIR)/$(PROJECT).elf

README.md

 You will also need to install some program to flash the firmware.  The simplest
 option is [dfu-util][], as it requires no extra hardware (though if upgrading
 from versions older than 1.2.0, either the Boot switch must be installed or two
-pads must be bridged).  If you prefer to use SWD, you could also use [stlink][]
-or [OpenOCD][].
+pads must be bridged).  If you prefer to use SWD, you could also use
+[stlink][], [OpenOCD][], or a [Black Magic Probe][].
 
 [toolchain]: https://launchpad.net/gcc-arm-embedded
 [dfu-util]: http://dfu-util.sourceforge.net/
 [stlink]: https://github.com/texane/stlink
 [OpenOCD]: http://openocd.org/
+[Black Magic Probe]: https://github.com/blacksphere/blackmagic/wiki
 
 ## Compiling
 
 
     $ make flash-openocd-stlink
 
+### Black Magic Probe
+
+Black Magic Probe debuggers can be used to flash the firmware as well.  This
+can be easily done as follows:
+
+    $ make flash-bmp
+
 ## Usage
 
 After first flashing the PD Buddy Sink, the device has no configuration.  To