add STM32F405 machine/runtime, and new board/target feather-stm32f405

targets/feather-stm32f405.json

@@ -0,0 +1,13 @@
+{
+  "inherits": ["cortex-m4"],
+  "build-tags": ["feather_stm32f405", "stm32f405", "stm32f4", "stm32"],
+  "cflags": [
+    "-Qunused-arguments"
+  ],
+  "linkerscript": "targets/stm32f405.ld",
+  "extra-files": [
+    "src/device/stm32/stm32f405.s"
+  ],
+  "flash-method": "command",
+  "flash-command": "dfu-util -a 0 --dfuse-address 0x08000000 -D {hex}"
+}

targets/stm32f405.ld

@@ -0,0 +1,9 @@
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x08000000, LENGTH = 1M
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 128K
+}
+
+_stack_size = 4K;
+
+INCLUDE "targets/arm.ld"