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/uf2format.h

@@ -0,0 +1,120 @@
+#ifndef UF2FORMAT_H
+#define UF2FORMAT_H 1
+
+#include <stdint.h>
+#include <stdbool.h>
+
+// All entries are little endian.
+
+// if you increase that, you will also need to update the linker script file
+#ifdef SAMD21
+#define APP_START_ADDRESS 0x00002000
+#define UF2_FAMILY 0x68ed2b88
+#endif
+#ifdef SAMD51
+#define APP_START_ADDRESS 0x00004000
+#define UF2_FAMILY 0x55114460
+#endif
+
+#define UF2_MAGIC_START0 0x0A324655UL // "UF2\n"
+#define UF2_MAGIC_START1 0x9E5D5157UL // Randomly selected
+#define UF2_MAGIC_END 0x0AB16F30UL    // Ditto
+
+// If set, the block is "comment" and should not be flashed to the device
+#define UF2_FLAG_NOFLASH 0x00000001
+#define UF2_FLAG_FAMILYID_PRESENT 0x00002000
+
+#define UF2_IS_MY_FAMILY(bl)                                                                       \
+    (((bl)->flags & UF2_FLAG_FAMILYID_PRESENT) == 0 || (bl)->familyID == UF2_FAMILY)
+
+typedef struct {
+    // 32 byte header
+    uint32_t magicStart0;
+    uint32_t magicStart1;
+    uint32_t flags;
+    uint32_t targetAddr;
+    uint32_t payloadSize;
+    uint32_t blockNo;
+    uint32_t numBlocks;
+    uint32_t familyID;
+
+    // raw data;
+    uint8_t data[476];
+
+    // store magic also at the end to limit damage from partial block reads
+    uint32_t magicEnd;
+} UF2_Block;
+
+typedef struct {
+    uint8_t version;
+    uint8_t ep_in;
+    uint8_t ep_out;
+    uint8_t reserved0;
+    uint32_t cbw_tag;
+    uint32_t blocks_remaining;
+    uint8_t *buffer;
+} UF2_HandoverArgs;
+
+typedef void (*UF2_MSC_Handover_Handler)(UF2_HandoverArgs *handover);
+typedef void (*UF2_HID_Handover_Handler)(int ep);
+
+// this is required to be exactly 16 bytes long by the linker script
+typedef struct {
+    const uint32_t *config_data;
+    UF2_HID_Handover_Handler handoverHID;
+    UF2_MSC_Handover_Handler handoverMSC;
+    const char *info_uf2;
+} UF2_BInfo;
+
+#define UF2_BINFO ((UF2_BInfo *)(APP_START_ADDRESS - sizeof(UF2_BInfo)))
+
+static inline bool is_uf2_block(void *data) {
+    UF2_Block *bl = (UF2_Block *)data;
+    return bl->magicStart0 == UF2_MAGIC_START0 && bl->magicStart1 == UF2_MAGIC_START1 &&
+           bl->magicEnd == UF2_MAGIC_END;
+}
+
+static inline bool in_uf2_bootloader_space(const void *addr) {
+    return 0xb4 <= (uint32_t)addr && (uint32_t)addr < APP_START_ADDRESS;
+}
+
+static inline const char *uf2_info(void) {
+    if (in_uf2_bootloader_space(UF2_BINFO->info_uf2))
+        return UF2_BINFO->info_uf2;
+    return "N/A";
+}
+
+#ifdef UF2_DEFINE_HANDOVER
+static inline void hf2_handover(uint8_t ep) {
+    const char *board_info = UF2_BINFO->info_uf2;
+    UF2_HID_Handover_Handler fn = UF2_BINFO->handoverHID;
+
+    if (in_uf2_bootloader_space(board_info) && in_uf2_bootloader_space((const void *)fn) &&
+        ((uint32_t)fn & 1)) {
+        // Pass control to bootloader; never returns
+        fn(ep & 0xf);
+    }
+}
+
+// the ep_in/ep_out are without the 0x80 mask
+// cbw_tag is in the same bit format as it came
+static inline void check_uf2_handover(uint8_t *buffer, uint32_t blocks_remaining, uint8_t ep_in,
+                                      uint8_t ep_out, uint32_t cbw_tag) {
+    if (!is_uf2_block(buffer))
+        return;
+
+    const char *board_info = UF2_BINFO->info_uf2;
+    UF2_MSC_Handover_Handler fn = UF2_BINFO->handoverMSC;
+
+    if (in_uf2_bootloader_space(board_info) && in_uf2_bootloader_space((const void *)fn) &&
+        ((uint32_t)fn & 1)) {
+        UF2_HandoverArgs hand = {
+            1, ep_in, ep_out, 0, cbw_tag, blocks_remaining, buffer,
+        };
+        // Pass control to bootloader; never returns
+        fn(&hand);
+    }
+}
+#endif
+
+#endif