Refactor bootloader_jump() implementations (#15450)

* Refactor `bootloader_jump()` implementations

* Fix tests?

* Rename `atmel-samba` to `md-boot`

platforms/avr/bootloaders/dfu.c

@@ -0,0 +1,53 @@
+/* Copyright 2021 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "bootloader.h"
+
+#include <avr/wdt.h>
+#include <util/delay.h>
+
+#define FLASH_SIZE (FLASHEND + 1L)
+
+/** \brief Entering the Bootloader via Software
+ *
+ * http://www.fourwalledcubicle.com/files/LUFA/Doc/120730/html/_page__software_bootloader_start.html
+ */
+#define BOOTLOADER_RESET_KEY 0xB007B007
+uint32_t reset_key __attribute__((section(".noinit,\"aw\",@nobits;")));
+
+__attribute__((weak))
+void bootloader_jump(void) {
+    UDCON  = 1;
+    USBCON = (1 << FRZCLK);  // disable USB
+    UCSR1B = 0;
+    _delay_ms(5);  // 5 seems to work fine
+
+    // watchdog reset
+    reset_key = BOOTLOADER_RESET_KEY;
+    wdt_enable(WDTO_250MS);
+    for (;;)
+        ;
+}
+
+/* this runs before main() */
+void bootloader_jump_after_watchdog_reset(void) __attribute__((used, naked, section(".init3")));
+void bootloader_jump_after_watchdog_reset(void) {
+    if ((MCUSR & (1 << WDRF)) && reset_key == BOOTLOADER_RESET_KEY) {
+        reset_key = 0;
+
+        ((void (*)(void))((FLASH_SIZE - BOOTLOADER_SIZE) >> 1))();
+    }
+}