Comment 6 for bug 544555

You lack the package gfxboot in your host system - this will supply the mkbootmsg command and is a dependency for (re-)building the gfxboot-theme-ubuntu. (This should be pulled in by installation of the uck*.deb, but as you are using an unpackaged pre-release version this does not happen automatically).

As an alternative:

If you don't need to modify gfxboot (i.e. you stay with the one from the Live-CD) you can also
remove all the lines (starting at :
     # Create a temporary directory to assemble the gfxboot stuff in
and ending at:
    # Cleanup
    [ "$BUILD_DIR" != "/" ] && rm -rf "$BUILD_DIR"
) from ~/tmp/customization-scripts/customize_iso.
This will save some time, too.