sam: add support for Atmel SAMD21 based ItsyBitsy M0

Signed-off-by: Ron Evans <ron@hybridgroup.com>

targets/atsamd21g18.json

@@ -0,0 +1,15 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv6m-none-eabi",
+	"build-tags": ["atsamd21g18", "sam"],
+	"cflags": [
+		"--target=armv6m-none-eabi",
+		"-Qunused-arguments"
+	],
+	"ldflags": [
+		"-T", "targets/atsamd21g18.ld"
+	],
+	"extra-files": [
+		"src/device/sam/atsamd21g18a.s"
+	]
+}

targets/atsamd21g18.ld

@@ -0,0 +1,10 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x2000, LENGTH = 0x00040000-0x2000  /* First 8KB used by bootloader */
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x00008000
+}
+
+_stack_size = 2K;
+
+INCLUDE "targets/arm.ld"

targets/itsybitsy-m0.json

@@ -0,0 +1,5 @@
+{
+    "inherits": ["atsamd21g18"],
+    "build-tags": ["sam", "atsamd21g18a", "itsybitsy_m0"],
+    "flash": "bossac -d -i -e -w -v -R --offset=0x2000 {hex}"
+}