fmu-v2: add bootloader binary and SYS_BL_UPDATE param for BL updates

This allows a simple-to-use bootloader update mechanism to switch from
fmu-v2 to fmu-v3 target that has more flash.

src/modules/systemlib/system_params.c

@@ -267,3 +267,25 @@ PARAM_DEFINE_INT32(SYS_HAS_MAG, 1);
  * @group System
  */
 PARAM_DEFINE_INT32(SYS_HAS_BARO, 1);
+
+/**
+ * Bootloader update
+ *
+ * If enabled, update the bootloader on the next boot.
+ *
+ * WARNING: do not cut the power during an update process, otherwise you will
+ * have to recover using some alternative method (e.g. JTAG).
+ *
+ * Instructions:
+ * - Insert an SD card
+ * - Enable this parameter
+ * - Reboot the board (plug the power or send a reboot command)
+ * - Wait until the board comes back up (or at least 2 minutes)
+ * - If it does not come back, check the file bootlog.txt on the SD card
+ *
+ * @boolean
+ * @reboot_required true
+ *
+ * @group System
+ */
+PARAM_DEFINE_INT32(SYS_BL_UPDATE, 0);