Navit has not been successfully ported to TomTom yet. However, there is work in progress. I will scratch my experiences here and, hopefully, end up with some guide on porting Navit to TomTom.
Lots of information can be found at http://www.opentom.org.
TomTom provides a compiler toolchain which is recommended for targetting TomTom devices. It can be downloaded from TomTom's website: http://www.tomtom.com/page.php?Page=gpl
Several libraries need are needed by Navit and must be ported as well.