bl_update: Allow booting from SD card (without forced BL copy to ROMFS)

ROMFS/px4fmu_common/init.d/rcS

@@ -613,7 +613,9 @@ else
 		. ${R}etc/init.d/rc.autostart.post
 	fi
 
-
+	#
+	# Bootloader upgrade (ROMFS)
+	#
 	set BOARD_BOOTLOADER_UPGRADE ${R}etc/init.d/rc.board_bootloader_upgrade
 	if [ -f $BOARD_BOOTLOADER_UPGRADE ]
 	then
@@ -621,6 +623,22 @@ else
 	fi
 	unset BOARD_BOOTLOADER_UPGRADE
 
+	#
+	# Bootloader upgrade (SD card)
+	#
+	if param compare -s SYS_BL_UPDATE 2
+	then
+		if [ -f "/fs/microsd/bl/bl.bin" ]
+		then
+			param set SYS_BL_UPDATE 0
+			param save
+			echo "bootloader update..."
+			bl_update "/fs/microsd/bl/bl.bin"
+			echo "bootloader update done, rebooting"
+			reboot
+		fi
+	fi
+
 	#
 	# Check if UAVCAN is enabled, default to it for ESCs.
 	#