Add UF2 bootloader for USB firmware flashing

Vendored and trimmed from microsoft/uf2-samdx1, adapted to the actual
ATSAMD21G17D (128 KiB flash / 16 KiB RAM), fixing the flash/RAM-size
mismatch that the previously commented-out SAM-BA bootloader target
had (it assumed a 256 KiB SAMD21G18A). Builds as a standalone
PlatformIO environment (bootloader/platformio.ini), no python2/make
dependency. Compiles clean, fits in the 8 KiB bootloader region
(7292/8192 bytes). Not yet flashed/verified on real hardware.

bootloader/lib/samd21/samd21a/include/system_samd21.h

@@ -0,0 +1,47 @@
+/**
+ * \file
+ *
+ * \brief Low-level initialization functions called upon chip startup
+ *
+ * Copyright (c) 2016 Atmel Corporation,
+ *                    a wholly owned subsidiary of Microchip Technology Inc.
+ *
+ * \asf_license_start
+ *
+ * \page License
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the Licence at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * \asf_license_stop
+ *
+ */
+
+#ifndef _SYSTEM_SAMD21_H_INCLUDED_
+#define _SYSTEM_SAMD21_H_INCLUDED_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdint.h>
+
+extern uint32_t SystemCoreClock;   /*!< System Clock Frequency (Core Clock)  */
+
+void SystemInit(void);
+void SystemCoreClockUpdate(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SYSTEM_SAMD21_H_INCLUDED */