stm32: add nucleo-l031k6 support

Adds i2c for all L0 series

UART, Blinky (LED) and i2c tested

targets/cortex-m0.json

@@ -0,0 +1,8 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv6m-none-eabi",
+	"cflags": [
+		"--target=armv6m-none-eabi",
+		"-Qunused-arguments"
+	]
+}

targets/nucleo-l031k6.json

@@ -0,0 +1,11 @@
+{
+    "inherits": ["cortex-m0"],
+    "build-tags": ["nucleol031k6", "stm32l031", "stm32l0x1", "stm32l0", "stm32"],
+    "linkerscript": "targets/stm32l031k6.ld",
+    "extra-files": [
+        "src/device/stm32/stm32l0x1.s"
+    ],
+    "flash-method": "openocd",
+    "openocd-interface": "stlink",
+    "openocd-target": "stm32l0"
+}

targets/stm32l031k6.ld

@@ -0,0 +1,10 @@
+
+MEMORY
+{
+  FLASH_TEXT (rx) : ORIGIN = 0x08000000, LENGTH = 32K
+  RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 8K
+}
+
+_stack_size = 2K;
+
+INCLUDE "targets/arm.ld"