The Tagged Boot Image format is what Nilo uses as it's native format. This is also the same format as used by Etherboot, and Netboot.
A boot image is made using the makebooti program. Makebooti is an evolution of mknbi-linux and mknbi-dos. The mknbi program are tottally seperate sources, with OS specific options. The idea with makebooti is to make a single program that is more configurable. Part of this will be using the GNU binutils BFD library to analyse the original executable kernel in a portable fashion. The other parameters like RAM disk size and location will be supplied in an OS specfic config file.