Add mmoskal/uf2-stm32f103 bootloader support (#19594)

platforms/chibios/bootloaders/uf2boot.c

@@ -0,0 +1,23 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "bootloader.h"
+
+// From mmoskal/uf2-stm32f103's backup.c
+#define MAGIC_BOOT 0x544F4F42UL
+
+// defined by linker script
+extern uint32_t _board_magic_reg[];
+#define MAGIC_REG _board_magic_reg[0]
+
+void bootloader_jump(void) {
+    MAGIC_REG = MAGIC_BOOT;
+    NVIC_SystemReset();
+}
+
+void mcu_reset(void) {
+    NVIC_SystemReset();
+}
+
+/* not needed, no two-stage reset */
+void enter_bootloader_mode_if_requested(void) {}