There's a lot of good advice in here, and I don't really disagree with any of it. If you want an album to be mastered for commercial release, yes, even iTunes et al., I would suggest getting a real mastering house to do it. They have the experience of hundreds if not thousands of releases, not to mention the listening environment and gear to fully take advantage of that experience. Others have commented on this stuff plenty, so no need to go on about that.
If, however, you value fun and DIY over convenience and commercial sheen as I do, then I'll give my initial thoughts on mastering in the Kronos. I am not a mastering engineer, but after a fair amount of reading and a lot of listening, I did deliver both of my albums to the CD dupe plant by myself. Trust this, though: they would sound better if done by a pro.
First of all, the audio tools are in there. Are they replacing a System 6000 MKII or some unobtanium boutique analog gear? Probably not. But for audio correction, stereo and tonal balance, creative enhancement, leveling, and comparing tracks to make a cohesive album, there's no reason you can't do it in the Kronos' sequencer. I bet Bob Ludwig or Ted Jensen could get good results.
Here is one potential (though extremely full and possibly redundant) chain of processors that you might use on a particular track to get the most from it:
037: St. Tube PreAmp Modeling
010: Stereo Noise Reduction (or expansion, or even gating)
014: Stereo Exciter/Enhncr
100: Overb (or other reverb/ambience)
013: Stereo Master 3EQ (or para EQ, or even graphic EQ)
004: St.Multiband Compressor (possibly up to 3 instances for "finalizer-style" exp/comp/lim)
006: Multiband Limiter
008: Stereo Mastering Limiter
It's doubtful that you'd use all of these on a single track. The tools in the first half of this chain are definitely on the creative and very optional side of things. The processors at the end of the chain here are closer to the classic EQ + Compression combo that you'd expect to use on most tracks. Also, the order of effects is certainly not set in stone, though you'll probably find that some conventions make sense such as putting your peak limiter last, putting any reverb after your gating (in the event that you're using these), etc.
The basic audio objectives for mastering a track are to normalize your audio, correct any problem frequencies, and fill the meters a bit without crushing your musical dynamics. Then you'll want to make the tracks sound cohesive as an album...if you had a really bright track and a really dull track, you'd probably want to compromise somewhere in the middle on each of them. You'd also want tracks to be generally similar in levels so that you don't have people repeatedly reaching for the volume knob while listening to your album. You want your tracks to sound good on a large variety of playback devices, from cars to earbuds to home theaters; your listeners should be able to hear all your instruments and parts. This all has to be carefully balanced with your artistic goals for your album's flow and mood.
How do you get there? Listening. That's the key. Listen to your favorite albums on the same system that you record with and try to analyze why they sound that way. Read about and play with each of the effects above until you find how to get those sounds. Then listen to the result on several different playback systems. Realize that an engineer has put as much time into turning knobs and listening as you have practicing and playing.
As musicians we take years to develop our playing technique and rigs to get the best possible sound out of our particular instrument, often for playing solo (that's what most of us do most of the time). It takes some discipline to fit in with a band. It's another level of discipline to record and mix such that each instrument gets its own space, and the compromises on individual instruments become larger. When we get to mastering, this really is the bird's-eye view. Don't be the drummer that mixes and masters his own album with cannon drums that are louder than the singer, or the guitarist that likes the pants-flapping bass out of his 4x12 so much that you try to duplicate that on record at all costs, drowning virtually everything else.
But I digress...I said I wasn't gonna say all that stuff...
After you've gotten your tracks close to what you want by repeatedly burning CDs on your Kronos and comparing them to your favorite albums on various systems, you really do have a self-mastered album, and can give the CD to anyone to listen to. What you don't have is customized gaps between tracks, CD text, ISRC codes, a UPC symbol and all that other stuff that you might get from a professionally mastered CD and/or duplication facility. Do these things matter to you? Are you selling this commercially?
In summation, yes, you can do the audio portion of mastering within the Kronos; at least the tools themselves won't be the limiting factor...
One last bit of advice: if you are getting your stuff professionally mastered, don't do much of this stuff on the version you send to them. Just give your best mixes. You're only crippling their ability to enhance your material by processing -- especially compressing -- your main bus.