Being familiar with RTOS's is certainly useful, since there's a decent chance that your future automotive/aero/industrial job uses one.
For a reference of stuff to learn about, go grab the manuals (usually massive) for a recent embedded CPU and see what common topics are foreign to you. Just reading the TOC should prompt some ideas. You may not be applying for one of the lower level "board bring-up" type jobs, but that team will be delivering you libraries that assumes familiarity with IRQs, timers, memory paging, processor modes, DMA, lots of I/O types, etc.
For a reference of stuff to learn about, go grab the manuals (usually massive) for a recent embedded CPU and see what common topics are foreign to you. Just reading the TOC should prompt some ideas. You may not be applying for one of the lower level "board bring-up" type jobs, but that team will be delivering you libraries that assumes familiarity with IRQs, timers, memory paging, processor modes, DMA, lots of I/O types, etc.