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
Actually, putting a link from [ to test in that image will fix the
Fixed in rawhide.