on boot AWK prints an error: awk: (FILENAME=/proc/mounts FNR=6) Error writing to standard output (Invalid argument) for some details please look here: http://forums.gentoo.org/viewtopic-t-429204-highlight-mtab+awk.html
no real info here ... do not post links to forums, post all relevant information into this bug report `emerge info` and the actual boot up output would be a good start
The error seems to come from a softlink of /etc/mtab to /proc/mounts. I have no idea where this came from. An easy fix is to remerge baselayout. This makes the error go away and the root (/) fs is mounted as it should.