Depends on device.
On devices like mi2 where its possible to boot unsigned bootloader we flash an "hacked" LK as main bootloader. On other devices the "hacked lk" will be flashed on boot. The bootloader thinks its booting the kernel but it boots a bootloader that boots grub
Grub is better than root (@
M1cha), it can have fun with ramdisk (and all your files) and Tracy will do some magic tricks to make multiboot loading partitions from sdcard. You will use folders (system data cache) and boot.IMG, firmware.IMG recovery.IMG from sdcard for each (IMG files if needed) system. No need to create IMG files for partitions.