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/inc/preprocessor/stringz.h

@@ -0,0 +1,67 @@
+/* ----------------------------------------------------------------------------
+ *         SAM Software Package License
+ * ----------------------------------------------------------------------------
+ * Copyright (c) 2011-2014, Atmel Corporation
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following condition is met:
+ *
+ * Redistributions of source code must retain the above copyright notice,
+ * this list of conditions and the disclaimer below.
+ *
+ * Atmel's name may not be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * DISCLAIMER: THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
+ * DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA,
+ * OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+ * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ * ----------------------------------------------------------------------------
+ */
+
+#ifndef _STRINGZ_H_
+#define _STRINGZ_H_
+
+/**
+ * \defgroup group_sam0_utils_stringz Preprocessor - Stringize
+ *
+ * \ingroup group_sam0_utils
+ *
+ * @{
+ */
+
+/** \brief Stringize.
+ *
+ * Stringize a preprocessing token, this token being allowed to be \#defined.
+ *
+ * May be used only within macros with the token passed as an argument if the
+ * token is \#defined.
+ *
+ * For example, writing STRINGZ(PIN) within a macro \#defined by PIN_NAME(PIN)
+ * and invoked as PIN_NAME(PIN0) with PIN0 \#defined as A0 is equivalent to
+ * writing "A0".
+ */
+#define STRINGZ(x)                                #x
+
+/** \brief Absolute stringize.
+ *
+ * Stringize a preprocessing token, this token being allowed to be \#defined.
+ *
+ * No restriction of use if the token is \#defined.
+ *
+ * For example, writing ASTRINGZ(PIN0) anywhere with PIN0 \#defined as A0 is
+ * equivalent to writing "A0".
+ */
+#define ASTRINGZ(x)                               STRINGZ(x)
+
+/** @} */
+
+#endif  // _STRINGZ_H_