machine/atsamd51: add support for ATSAMD51 processor using Adafruit ItsyBitsy-M4 board

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

targets/atsamd51.ld

@@ -0,0 +1,10 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x4000, LENGTH = 0x00080000-0x4000  /* First 16KB used by bootloader */
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x00030000
+}
+
+_stack_size = 4K;
+
+INCLUDE "targets/arm.ld"

targets/atsamd51g19a.json

@@ -0,0 +1,15 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv7em-none-eabi",
+	"build-tags": ["atsamd51g19", "atsamd51", "sam"],
+	"cflags": [
+		"--target=armv7em-none-eabi",
+		"-Qunused-arguments"
+	],
+	"ldflags": [
+		"-T", "targets/atsamd51.ld"
+	],
+	"extra-files": [
+		"src/device/sam/atsamd51g19a.s"
+	]
+}

targets/itsybitsy-m4.json

@@ -0,0 +1,5 @@
+{
+    "inherits": ["atsamd51g19a"],
+    "build-tags": ["sam", "atsamd51g19a", "itsybitsy_m4"],
+    "flash": "bossac -d -i -e -w -v -R --offset=0x4000 {bin}"
+}