AL2 refers to the SOC, which in RB4011's case is AL21400 by Annapurna Labs.
On a CCR10xx for example the firmware type is tilegx - which refers to TILE-Gx SOC by Tilera.
There is no firmware to download separately.
The RouterBOOT firmware has been merged into the RouterOS packages for years now.
When you install a newer RouterOS version, if there are changes to the RouterBOOT firmware, it will automatically get installed.
Keep in mind that you have to manually hit upgrade on System > RouterBOARD menu and then reboot, in order to actually update the RouterBOOT firmware.
https://help.mikrotik.com/docs/display/ROS/RouterBOOT