GRUB is the boot-loader most commonly used with L4. The boot-loader is used to load the binary images of the system to run into main-memory. The source can be (in the case of GRUB) hard-disk, floppy, network. CD-ROM, and others. Experience showed that, if you have the resources, booting via network is fastest for development cycles. You can build your software, copy it into a directory accessible to the TFTP Server, and then load the images via TFTP over the network.
GRUB supports the multiboot standard, that is, it can load multiple binaries into main-memory and then start executing one of them. This is used by the L4 System: All the binaries are loaded into main memory and then bootstrap is executed which uses the multiboot information provided by GRUB to find and start the other binaries. To know which files to load, GRUB reads a file called menu.lst.