This isn't really a mkbootdisk problem, but I didn't find a section for boot image problems. /bin/init on rescue.img uses a [] construct which is not supported by the ash shell on the disk, and filesystem unmounting fails. I think you will have to add /bin/test if you want that functionality with ash. Also, for reasons I don't understand, when exiting the shell (which is supposed to halt the machine) the (ATX) machine is not bootable via the reset switch or front power switch. The ATX/motherboard interface is being mucked up somehow. This doesn't happen when doing a shutdown from an installed 6.0 system.
Actually, putting a link from [ to test in that image will fix the syntax error.
Fixed in rawhide.