change default flash method to DFU (using dfu-util over USB)

targets/feather-stm32f405.json

@@ -10,6 +10,8 @@
   "extra-files": [
     "src/device/stm32/stm32f405.s"
   ],
+  "flash-method": "command",
+  "flash-command": "dfu-util --alt 0 --dfuse-address 0x08000000 --download {bin}",
   "openocd-transport": "swd",
   "openocd-interface": "jlink",
   "openocd-target": "stm32f4x"