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/samd21.h

@@ -0,0 +1,69 @@
+/**
+ * \file
+ *
+ * \brief Top header file for SAMD21
+ *
+ * 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 _SAMD21_
+#define _SAMD21_
+
+/**
+ * \defgroup SAMD21_definitions SAMD21 Device Definitions
+ * \brief SAMD21 CMSIS Definitions.
+ */
+
+#if   defined(__SAMD21E15A__) || defined(__ATSAMD21E15A__)
+  #include "samd21e15a.h"
+#elif defined(__SAMD21E16A__) || defined(__ATSAMD21E16A__)
+  #include "samd21e16a.h"
+#elif defined(__SAMD21E17A__) || defined(__ATSAMD21E17A__)
+  #include "samd21e17a.h"
+#elif defined(__SAMD21E18A__) || defined(__ATSAMD21E18A__)
+  #include "samd21e18a.h"
+#elif defined(__SAMD21G15A__) || defined(__ATSAMD21G15A__)
+  #include "samd21g15a.h"
+#elif defined(__SAMD21G16A__) || defined(__ATSAMD21G16A__)
+  #include "samd21g16a.h"
+#elif defined(__SAMD21G17A__) || defined(__ATSAMD21G17A__)
+  #include "samd21g17a.h"
+#elif defined(__SAMD21G17AU__) || defined(__ATSAMD21G17AU__)
+  #include "samd21g17au.h"
+#elif defined(__SAMD21G18A__) || defined(__ATSAMD21G18A__)
+  #include "samd21g18a.h"
+#elif defined(__SAMD21G18AU__) || defined(__ATSAMD21G18AU__)
+  #include "samd21g18au.h"
+#elif defined(__SAMD21J15A__) || defined(__ATSAMD21J15A__)
+  #include "samd21j15a.h"
+#elif defined(__SAMD21J16A__) || defined(__ATSAMD21J16A__)
+  #include "samd21j16a.h"
+#elif defined(__SAMD21J17A__) || defined(__ATSAMD21J17A__)
+  #include "samd21j17a.h"
+#elif defined(__SAMD21J18A__) || defined(__ATSAMD21J18A__)
+  #include "samd21j18a.h"
+#else
+  #error Library does not support the specified device.
+#endif
+
+#endif /* _SAMD21_ */