Fixup bootloaders. (#16256)

keyboards/matrix/m20add/m20add.c

@@ -59,3 +59,8 @@ void shutdown_user(void)
     // set the magic number for resetting to the bootloader
     *(uint32_t *)(&(RTCD1.rtc->BKP0R)) = REBOOT_MAGIC;
 }
+
+void bootloader_jump(void) {
+    shutdown_user();
+    NVIC_SystemReset();
+}

keyboards/matrix/m20add/rules.mk

@@ -10,6 +10,9 @@ MCU_LDSCRIPT = m20add_boot
 #  or <this_dir>/boards
 BOARD = ST_NUCLEO64_F411RE
 
+# Bootloader selection
+BOOTLOADER = custom
+
 # Build Options
 #   change yes to no to disable
 #