swd: Add debug screipt for GDB, and openocd command line

00-README-DEBUG

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+Run OpenCD:
+~/install/bin/openocd -s ~/install/share/openocd/scripts/ -f interface/cmsis-dap.cfg -f target/at32f4xx.cfg
+
+In another window, run GDB (note elf image name is hardcoded in the script):
+gdb-multiarch -x scripts/gdb-commands
+
+Telnet commands:
+telnet localhost 4444
+mdb 0x1fffc000 512 # Read USD on 435
+mdb 0x1ffff800 512 # Read USD on 415
+at32f4xx mass_erase
+at32f4xx usd_erase        # NB. Enables RDP!
+at32f4xx usd_write 0 0xa5 # Disable RDP
+flash write_image [erase] flashfloppy/out/at32f435/debug/floppy/target.hex
+reset run|halt|init

scripts/gdb-commands

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+file /home/keir/flashfloppy/out/at32f435/debug/floppy/target.elf
+target extended-remote localhost:3333
+# Reset the target and call its init script
+monitor reset init
+# Halt the target. The init script should halt the target, but just in case
+monitor halt